How Did The Blind Man Catch This Cheating Cashier?

Algebra Level 2

In the United States, the dollar bills in general circulation come in denominations of $1, $5, $10, $20, $50, $100. These bills are all the same size, shape and texture, which makes it hard for a blind person, like Matthew, to tell the difference between the bills.

Matthew went shopping at a toy store for his nephew’s birthday. He bought a Spiderman action figure that cost $25, a LEGO City Light Repair Truck for $13, and a Despicable Me 2 DVD for $20. He went up to the counter and handed over a new crisp $100 bill to pay for these purchases. The teller handed him some number of notes in return, and Matthew knew for certain that his change was incorrect.

Which of the following is a possibility for the number of bills that the teller handed to Matthew?

3 5 6 4

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38 solutions

Arpit Sah
Jan 18, 2014

Total cost of items bought by Matthew = $58

Change that should have been given to Matthew = $42

If the change was correct, then the teller could give him 4, 5 or 6 bills.

$(20 + 10 + 10 + 1 + 1)= $42 (5 bills)

$(20 + 20 + 1 + 1) = $42 (4 bills)

$(1 + 1 + 5 + 5 + 10 + 20) = $42 (6 bills)

The question says that the change given was incorrect.

Therefore, the change given to him must not be $42.

Using 3 bills we cannot total up $42.

Hence , the possibility that the number of bills that the teller handed to Matthew is 3 \boxed{3} .

the shop keeper has to return $42 which can not be returned by 3 notes as we need atleast 4 note(20+20+1+1)

Ashutosh Kumar - 7 years, 4 months ago

this is great

Lubega Jonah Mukisa - 7 years, 4 months ago

same way!!!

Prasad Nikam - 7 years, 4 months ago

wow

Cherry Nicer - 7 years, 4 months ago

gud one

aishu .. - 7 years, 4 months ago

good

Muhammad Khan - 7 years, 4 months ago

exactly!

Lakshmi Tumati - 7 years, 4 months ago

Right

Khondoker Hasib - 7 years, 4 months ago

is it 2 DVD or 1 DVD??

Fahim Rahman - 7 years, 4 months ago

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1

Callum Farnden - 7 years, 3 months ago

hahaha thinking of all the possibilities yet the answer was supposed to be the wrong one hahaha because the question said the change was incorrect haha

Jep Araza - 7 years, 3 months ago

i think there's problem in understanding the question. Total cost of items bought by Matthew = $68(25$+13$+2*20$) because he bought despicable me (2) dvd )

Change that should have been given to Matthew = $22

If the change was correct, then the teller could give him 5, 4 or 3 bills.

$(20 + 1 + 1)= $22 ( 3bills)

$(10 +10 + 1 + 1) = $22 (4 bills)

$(1 + 1 +5+5+10) = $42 (5 bills)

The question says that the change given was incorrect.

Therefore, the change given to him must not be $22.

Using 2 bills we cannot total up $42.

Hence , the possibility that the number of bills that the teller handed to Matthew is 2

Sälmän Rähmän - 7 years, 4 months ago

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And that is why you should pay attention to language. If he bought two DVDs, the word would be 'DVDs', not 'DVD'. Also the question states that 'a DVD' was bought.

Zed En - 7 years, 4 months ago

The Despicable Me 2 is a movie by itself not 2 dvds

Tony Zhao - 7 years, 4 months ago

Well he bought 'Despicable Me 2 ' dvd and not 2 'Despicable Me' dvd's...

Anuj Modi - 7 years, 3 months ago

underline the word certain .....zz

Viral Gala - 7 years, 3 months ago

The dvd is of the second part dude..but your opinion cannot be ruled out..good thinking though!! :)

Parth Viradia - 7 years, 4 months ago

Mind it Salman Buddy, they didn't say 20 dollar for each DVD. All they mentioned was 2 DVDs for 20$ !!

Garima Garima - 7 years, 4 months ago

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no..Garima Dear..its Despicable me 2nd part dvd...(a despicable me 2 DVD for 20)...single DVD...of the 2nd part of the movie Despicable me..;)

Jatin Taneja - 7 years, 4 months ago

not mentioned cost of dvd is 20 here mentioned cost of 2 dvds is 20 so u cannot sum up 2X20 so ur formation is wrong

Prasanth Chowdary - 7 years, 4 months ago

I accidentally answered the opposite question there

Callum Farnden - 7 years, 3 months ago
David Kroell
Jan 19, 2014

you know that you are going to have two, one dollar bills no matter what.

since there is no such thing as a $40 bill then 3 bills must be the only one which is impossible. Unless he made his own $40 bill ;)

Great solution!

Ryan E - 7 years, 4 months ago

i think there's problem in understanding the question. Total cost of items bought by Matthew = $68(25$+13$+2*20$) because he bought despicable me (2) dvd )

Change that should have been given to Matthew = $22

If the change was correct, then the teller could give him 5, 4 or 3 bills.

$(20 + 1 + 1)= $22 ( 3bills)

$(10 +10 + 1 + 1) = $22 (4 bills)

$(1 + 1 +5+5+10) = $42 (5 bills)

The question says that the change given was incorrect.

Therefore, the change given to him must not be $22.

Using 2 bills we cannot total up $42.

Hence , the possibility that the number of bills that the teller handed to Matthew is 2

Sälmän Rähmän - 7 years, 4 months ago

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You know, the 2 doesn't refer to the number of the DVD he bought. It's part of the movie's title.

Andiel Kyxz - 7 years, 4 months ago
Arjun Kuruba
Jan 18, 2014

the amount Matthew had to receive back were (40 +2)$. The 2$ can be given only in two notes of 1$. The remaining balance is 40$. Since there is no note for 40$ itself it has to be given as a combination of the available denominations. Hence the minimum number of notes given cannot be 3. hence the answer. :)

great soln

abhishek agrawal - 7 years, 4 months ago

i think there's problem in understanding the question. Total cost of items bought by Matthew = $68(25$+13$+2*20$) because he bought despicable me (2) dvd )

Change that should have been given to Matthew = $22

If the change was correct, then the teller could give him 5, 4 or 3 bills.

$(20 + 1 + 1)= $22 ( 3bills)

$(10 +10 + 1 + 1) = $22 (4 bills)

$(1 + 1 +5+5+10) = $42 (5 bills)

The question says that the change given was incorrect.

Therefore, the change given to him must not be $22.

Using 2 bills we cannot total up $42.

Hence , the possibility that the number of bills that the teller handed to Matthew is 2

Sälmän Rähmän - 7 years, 4 months ago
Aman Jaiswal
Jan 18, 2014

I got i this way.

The total shopping was of $58 and he should have gotten a change of $42.

So this means he could have received money in he following denominations $1,$5,$10 and $20.

Receiving a change of $42 is impossible if he gets only 3 notes.

Hope this helps...cheers!!!!

So the answer is 3

Aman Jaiswal - 7 years, 4 months ago
John M.
Jan 19, 2014

$25 Spiderman $13 LEGO CLRT + $20 Despicable


$58

$100-$58=$42.

The cashier owed $42 change to Matthew.

Trying the lowest of these combinations, it's obvious, that $20, $20, and $2 combination is invalid since $2 dollars are not valid. Any other bill combination will deem invalid also.

Legal bills: $1, $5, $10, $20, $50, $100. $50 is useless, $10 too low, so 2 $20's makes sense, but nothing to top off with.

Therefore, 3 is a possibility.

Amith Rc
Jan 18, 2014

A change of $42 to be tendered can be done by:

6 notes ($10 x 4 notes + $1 x 2 notes)

4 notes ($20 x 2 notes + $1 x 2 notes)

5 notes ($20 x 1 note + $10 x 2 notes + $1 x 2 notes)

But no combinations, can he tender the $42 change with 3 notes.

Daksh Shami
Jan 18, 2014

It is very simple. The person had to return change i.e. $42. Now we look at the minimum of the option i.e. 3. The person could have returned only $41 and he would have to give one more note to make it $42.

Hence the answer is 3

Isaac Mast
Jan 18, 2014

This is what the blind man bought for his nephew. Spiderman action figure $25 + Truck $13 + Despicable Me 2 $20 = For a total of $58 He handed the cashier a $100 bill. This question states that in general, bills will come in $1, $5, $10, $20, $50, and $100's. Therefor, when the teller gave him the least amount of bills, he would have received two $20's and two $1 bills, which is 4 bills.

Unfortunately you did not understand the question.. Answer is 3.. :-)

Snehal Shekatkar - 7 years, 4 months ago

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He understood it well. 4 is the minimum amount of bills Matthew would have gotten, so he could not get 3 bills.

Parth Kohli - 7 years, 4 months ago

Since that he has 100$ bill,

so the change=$10--$20-$25-$13=$42

so the probable change notes are=$1,$5,$10,$20 that is 4 of them as the change is < $50 and < $100

so by combination to fill up that $42 with bills= 4C1=4

thanks...

Since the change is $42 the cashier got to use his $1 notes twice and others me will manage. So there will be more than 2 notes for a correct cashier but since the present one is incorrect it should be three

42-cannot be formed by three notes

42-can be formed by 4 notes ($1,$1,$20,$20)

How is it that your answer is 4 ???

Priyanka Banerjee - 7 years, 4 months ago

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U r Right. but 2 is not any option.

Deepam Gupta - 7 years, 4 months ago

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The answer is 3, not 2.

Anish Puthuraya - 7 years, 4 months ago

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@Anish Puthuraya The answer should be 3, and not 4

Anand Mallaya - 7 years, 4 months ago

$42 can be made by 4 notes (1,1,20,20), by 5 notes(1,1,10,10,20), and by 6 notes(1,1,10,10,10,10). But not by 3 notes (1,20,20..????). So Matt knew that his change was wrong when the teller handed him 3 notes. So answer is 3.

Lokesh Gupta - 7 years, 4 months ago

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Exactly...I also put 3 due to the same logic and was marked incorrect....

Eddie The Head - 7 years, 4 months ago

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Hey I answered 3 and got my points....the problem has been corrected....

Røhît Várânãsï - 7 years, 4 months ago

Me too

Thaddeus Abiy - 7 years, 4 months ago

By the same logic I marked 3 just to see that somehow it is incorrect.. I don't understand given solution.. :(

Snehal Shekatkar - 7 years, 4 months ago

The answer is 3. I lost my ratings too! Want it back!

Anish Puthuraya - 7 years, 4 months ago

Same goes for me.

hubert światkiewicz - 7 years, 4 months ago

By no means 4 could be the answer. I was dumb struck to see 3 incorrect.

Durga Sai Prasad - 7 years, 4 months ago

This problem needs to be corrected, or taken down.

Matthew Yu - 7 years, 4 months ago

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I want my rating points back.. :)

Snehal Shekatkar - 7 years, 4 months ago

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and the sum rating has gone so high because almost everyone who tried it has got it wrong.... The sum should be taken down...

Prerona Chatterjee - 7 years, 4 months ago

me tooo!!!!

Prerona Chatterjee - 7 years, 4 months ago

I also answered 4. But why is it 3 ?

Kou$htav Chakrabarty - 7 years, 4 months ago

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You first explain why is it 4? :-)

Snehal Shekatkar - 7 years, 4 months ago

I answered 3 :(

Antonio Valente Macarilay - 7 years, 4 months ago

With 4 bills ==> 20 + 20 + 1 + 1 With 5 bills ==> 20 + 10 + 10 + 1 + 1 With 6 bills ==> 10 + 10 + 10 + 10 + 1 + 1

With 3 bills ==> No three of the bills can add up to 42. And, so the answer has to be 3.

Shamik Banerjee - 7 years, 4 months ago

So, has anyone posted a dispute yet?

Mursalin Habib - 7 years, 4 months ago

Since the man gave the cashier a $100 bill, the amount he should get back is $100-$58=$42. 42=1+1+10+20+10 (5 notes used) 42=1+1+20+20 (4 notes used) 42=1+1+5+5+10+20 (6 notes used) Also, 42 can't be formed with 3 notes. Thus 3 is a possibility.

Umang Rohani - 7 years, 4 months ago

4C1 is the number of ways of selecting one 1 from 4 items right...there is no $ 42 bill...so if the cashier give him a single bill(any denomination)..then surely matthew can tell(and this 4C1 that is 4 (your answer is the NUMBER OF WAYS of giving matthew the wrong change by just giving him a SINGLE bill))....and similarly cashier can give him any 2 bills (that wouldn't also add up to 42 as well)...(BUT THEY ARE NOT ASKING for the possible combinations of the 2 bills AS WELL).............

(Taking one,two or three bills at a time of the given denominations will never add up to 42)

in simple words---if cashier gives Matthew a single bill or any two bills or any three bills --he can surely tell that he is being cheated...but if cashiers gives him 4,5 or 6 (or above) number of bills he cannot tell if he is being cheated...

For Example -- if cashier gives him 4 bills---- (20+20+1+1). = 42(correct change)....and also (5+5+5+5)=20(incorrect change)..or (1+1+1+1)..and so on....

so he can't tell....

but in case if is handled with one,two or three bills...then HE CAN SURELY TELL about the cheating....

so among the options 3 is correct.....because by using 4 ,5 and 6 bills at a time we can make a total of 42......

I hope u get it now....and.if not then read the question more carefully..one more time.....

Jatin Taneja - 7 years, 4 months ago

The question is, how Blind person came to know that there is some mistake ?

bill was $48 so change to be returned, was 52. And 52 can't be paid in 4 notes.

Raj Mishra - 7 years, 4 months ago

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bill was 58, not 48!!!!

Prerona Chatterjee - 7 years, 4 months ago

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sorry!! it's my mistake..!!

Raj Mishra - 7 years, 4 months ago
Aaditya Sah
Mar 4, 2014

6 bills 10,10,10,10,1,1 or 20,10,5,5,1,1 5 bills 20,10,10,1,1 4 bills 20,20,1,1 3 bills 20,20,1 not=42 in any case

Ivy Joy Barrion
Feb 12, 2014

Toys has total cost of $58 so he should be changed of $42. Means, it can be $20,$20,$1 or $1 or $20,$10,$10,$1&$1 or $10,$10,$10,$10,$1,&1. Or with 4,5 or 6 number of bills.

The Teller will never return $100 or $50 bills. So we remain with $1, $5, $10, $20. Matthew gets back $(100-58)=$42. So to get wrong return he must have got * 2 $20 bills * 1 $20 bill & 2 $10 bills and so on...... but the number of bills increases a lot So the minimum no. of bills he gets back is 2 but since there is no option for 2 , I chose 3.........

Vivian Sudhir
Feb 5, 2014

Total expense = 25 + 13 + 20 = 58 Amount Mathew should receive = 100 -58 = 42 There are many ways to make up this amount But you need at least 4 bills to do it. 20+20+1+1(4 bills), 20+10+10+1+1(5 bills), 20+10+5+5+1+1(6 bills),upto (5+5+5+5)+(5+5)+(5+5)+1+1(10 bills), and then you can break up the $5 into $1 bills. This will give 5+5+5+5+5+5+5+(1+1+1+1+1)+ 1+1(14 bills) and then on you add 4 bills for every $5 you break. The sequence from 10 bills becomes 14,18, 22, 26, 30, 34, 38, and the last is 42 where everything is in $1 bills. Therefore, the wrong number is 3.

Gaurav Agarwal
Feb 5, 2014

the ans is three because the minimum number of notes that the cashier can give is 4.... 2 of 20 and 2 of 2..... ;)

Prasun Biswas
Feb 3, 2014

The total cost for the purchases are = $ ( 25 + 20 + 13 ) = $ 58 =\$(25+20+13)=\$58

Now, since Matthew handed a $ 100 \$100 bill, so the cashier should give change = $ ( 100 58 ) = $ 42 =\$(100-58)=\$42

Now, the cashier can give the change at the very least using 4 bills ( 2 2 $ 20 \$20 bills and 2 2 $ 1 \$1 bills).

But there is no way to give change using only 3 bills, so the possible number of bills that the cashier gave to Matthew = 3 =\boxed{3}

Saikat Dey
Feb 1, 2014

The blind man's purchase add up to $58. So, the cashier must return him $42. Now, whatever be combination the cashier uses, he has to give two $1 notes to the blind man. So, there is no possibility of adding up to $42 using two $1 notes and any other denominations if only 1 more note is to be used. So, the cashier gave him 3 notes.

Bharath Bhat
Feb 1, 2014

Matthew purchased for a sum of 58$(25+13+20) and should receive 42$ from the keeper. the minimum number of bills that make up 42$ is 4 i.e 20+20+1+1. if Matthew is sure that, what he received is wrong, then the number of bills he has is less than what he must have had. so the only possibility is that the store keeper gave him only 3 bills.

Vaibhav Modi
Jan 31, 2014

Total cost is $58, Therefore 100-58=42 Possible combination of teller returning exact bills 1) 1+1+20+20 = 4 notes ; or 2) 1+1+10+10+20 = 5 notes ; or 3) 1+1+10+10+10+10 = 6 notes since Matthew knew this change was incorrect only possibility of teller giving 3 notes remain

Tallal Jamshed
Jan 30, 2014

total amount of the purchases is 58 the remaining money is 42 so the cashier can give him [ 4 notes of 10$ and 2 notes of 1$ that makes total of 6 notes and 42$]...or....[1 note of 20$ 2 notes of 10$ and 2 notes of 1$ that makes 5 notes and 42$]...or...[2 notes of 20$ and 2 notes of 1$ that makes 4 notes and 42$] but he cannot give 3 notes

Vinod Kumar
Jan 25, 2014

6 for $10+$10+$10+$10+$1+$1; 4 for $20+$20+$1+$1; 5 for $20+$10+$10+$1+$1; but none of the way is suitable for 3

Toufiq Islam Noor
Jan 23, 2014

Total cost is $58. So Matthew should get back $42.

Since U.S. dollar bills are $1, $5, $10, $20, $50, $100, two of the returned bills must be $1. To return remaining $40, the teller needs at least two bills of $20.

Therefore the minimum number of bills must be four.

So, there is a possibility that the teller handed 3 bills to Matthew.

Lloyd Fernandes
Jan 23, 2014

Balance Amount Left: 42$ Minimum number of notes required = 4 i.e.(20+20+1+1) Hence 3 is the incorrect number of notes.

Total spending = 25+13+20 = 58, Amount paid = 100, balance = 100 - 58 = 42, balance is incorrect. Bills Might have given are $1, $5, $10, $20, he was not tendering exact change. it might be $1, $10, $20 or $5, $10, $20 or $1, $5, $10 or $1, $5, $20, The number of bills = 3.

Shafeeq Malik
Jan 22, 2014

3

Rohit Bharti
Jan 22, 2014

minimum number of bills teller handed to the Matthew should be 4

Testuser Apps
Jan 22, 2014
  • Total Purchase Amount = $ 58 ;
  • Teller Should return = $ 42 ;

  • Possible combination of return amount : $1, $5, $10, $20, $50, $100

  • $1 + $1 + $20 + $20 = $42( possible) ; Option 4 is possible

  • $1 + $1 + $20 + $10 + $10 = $42 (possible) ; Option 5 is possible
  • $1 + $1 + $10 + $10 + $10 + $10 = $42 (possible) ; Option 6 is possible

  • but Teller cannot return $42 with sum of 3 combination. ;

  • Option 3 is not possible
Kshitij Onkar
Jan 21, 2014

teller needs to return bill 40, considering the available denominations it is not possible to return 3 bills which would amount 40 in total.

Rishabh Kalia
Jan 21, 2014

3, because it is only with 3 bills that there is no permutation of making the amount equal to the change,and since he was blind it is the only way he could have known.....

Yogesh Gupta
Jan 21, 2014

according to the currency denomination, having 3notes doesn't sumup to the left over amount of $42

Option 1 is correct: $1,$1,$10,$10,$20

Option 2 is correct: $1,$1,$10,$10,$10,$10

Option 3 is correct: $1,$1,$20,$20

Shreedhar Kodate
Jan 21, 2014

We have spent 58 out of 100 so we must get in return 42. Options are 3,4,5,6 bills. One of these is incorrect combination of bills. ---for 4 ->2 x $1 + 2 x $20---- ---for 5 ->2 x $1 + 1 x $20 + 2 x $10---- ---for 6 ->2 x $1 + 4 x $10---- and with only 3 bills Matthew cannot get $42 in change. Hence answer is 3.

Saurabh G.
Jan 21, 2014

the balance is $42... which means two of the bills will be $1 bills... this leaves us with $40... it's easy to figure out combinations that will give us this sum... (remember that we only have [n-2] possibilities remaining).

Rahul Nainwal
Jan 20, 2014

as the mathew has purchase only three items.it is obvious that he will get three bills

Kalyan Pakala
Jan 20, 2014

The change to be returned is 42$. The change can be in denomination of 20 or lesser and in all the sets of 3 notes, it is impossible to make an exact of 42$!

Shibin Kuriakose
Jan 19, 2014

cost 58 , change = 42

6) 10 + 10 + 10 + 10+ 1 + 1

4) 20+ 20 + 1 + 1

5) 20 + 10 + 1 + 1

3 bills can never make $42 .

Marwa Ghallab
Jan 19, 2014

3

Why bother posting this

John M. - 7 years, 4 months ago
Arnab Acharya
Jan 19, 2014

Matthew made a total purchase of $ 20 + $ 25 + $ 13 = $ 58 \$20 + \$25 + \$13 = \$58

The teller was supposed to return him $ 100 $ 58 = $ 42 \$100 - \$58 = \$42

Now, with the available denominations it is only possible to form $42 in three ways :

  1. $ 20 × 2 + $ 1 × 2 \$20 \times 2 + \$1 \times 2 (4 bills)
  2. $ 10 × 4 + $ 1 × 2 \$10 \times 4 + \$1 \times 2 (6 bills)
  3. $ 5 × 8 + $ 1 × 2 \$5 \times 8 + \$1 \times 2 (10 bills)

Any number of bills handed to Matthew other than 4, 6 or 10 will certainly be an incorrect change.

Hence, 3 and 5 are the incorrect changes (and correct answers).

Errata : Excuse my stupidity, 5 5 bills( $ 20 , $ 10 , $ 10 , $ 1 , $ 1 \$20, \$10, \$10, \$1, \$1 ) can also be a possibility.

Leaving 3 \boxed{3} as the correct answer

Arnab Acharya - 7 years, 4 months ago
Sathi Nagi Reddy
Jan 18, 2014

As he purchased three items only, bill person at counter will give 3 bills only although he may give incorrect change

I think you misunderstood the question. Here, 'bills' mean the 'notes' themselves...

Røhît Várânãsï - 7 years, 4 months ago
Muhammad Usman
Jan 18, 2014

20+20+1=41 that is wrong so this is seller's option

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