Are you smarter than a fifth grader?

A 2009 digit number N N is multiplied by 54. The last two digits of this product are 68. When N N is multiplied with 46, find the last two digits.

Note- This question is taken from a class 5 Math talent exam paper, thus the title.


The answer is 32.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

7 solutions

Antony Diaz
Jun 4, 2014

We have a number a 1 a 2 a 3 . . . . a 2008 a 2009 { a }_{ 1 }{ a }_{ 2 }{ a }_{ 3 }....{ a }_{ 2008 }{ a }_{ 2009 } .

So, if we multiply a 2008 a 2009 x 54 = ( 10 a 2008 + a 2009 ) x 54 = . . 68 \overline { { a }_{ 2008 }{ a }_{ 2009 } } x54=(10{ a }_{ 2008 }+{ a }_{ 2009 })x54=..68 , we obtain that the solution are ( a 2008 , a 2009 ) = { ( 4 , 2 ) , ( 9 , 2 ) } ({ a }_{ 2008 },{ a }_{ 2009 })=\left\{ (4,2),(9,2) \right\} . So thats mean that: 42 X 46 = . . 32 92 X 46 = . . 32 \overline { 42 } X46=..32\\ \overline { 92 } X46=..32

how can a 5 grader solve like this

Avisek Agarwal - 7 years ago

Log in to reply

Bcoz it is simple multiplication thatz applied.Though it is a far cry its not out of syllabus

Chandrachur Banerjee - 7 years ago

Elegant! Dayum! XD

Krishna Ar - 7 years ago

how did yo got 42 and 92

komal kshirsagar - 7 years ago

We are told there is just one solution, so we needn't worry about multiple options.

Some number times 54 is 68. The last digit may be 2 (2 4=8) and since there is nothing to be carried and 54 2=208 which has no tens digit, the second to last digit may be 4 (4 4=16). 42 54=2268.

42*46=92+1840=1932. The answer is 32.

Jason Short - 5 years, 10 months ago
Zakaria Sellami
Jun 10, 2014

Let's call that number: a.
We have: 54 a 68 [ 100 ] 54a \equiv 68[100] which means 46 a 68 [ 100 ] -46a \equiv 68[100] hence 46 a 68 32 [ 100 ] 46a \equiv -68 \equiv 32[100]

Cool!!!!!!!!!!!!!!!!!

Krishna Ar - 6 years, 12 months ago

Yep, but not sure how to proceed as a fifth grader.

Zakaria Sellami - 6 years, 11 months ago
Suresh Bala
Sep 5, 2014

It can be done very quickly this way

Given that n × 54 n \times 54 when divided by 100 leaves a remainder 68

n × 46 \Rightarrow n \times -46 leaves a remainder 68

n × 46 \Rightarrow n \times 46 leaves a remainder - 68 i.e -68+100 = 32

I think this problem can be solved without any knowledge on number theory......how?????lets see!!!!!!!First........the last no of the given product is 8...4 can form this no. only after multiplication with a number ending with 2 or 7.......here lies the sums beauty!!!!!! '6'of the no 46 when multiplied with any of 2 or 7 yields the last number as 2....!!!!!!So we are confirmed that the last no. of the required product is 2............!!! now the second-last number of the given product is 6.........now let the second last no.of the given number be "x".....so 2 eqations may be formed
4x+(5.2)=(something)6 or 4x+(5.7)=(something)6 but second equation is not possibe as sum of a even and odd no.always yields an even no. so for first eqation the soln of "x"are 4 or 9 as the last digit of (5.2) is 0 so when multiplied by 46 yields the no.as 3........thats it!!!!!!!!!!!!!!!!!!!!!!

That's how I did it..

BIkash B - 7 years ago

i think there is a mistake. in your eqn 4x+5.7 . if your last digit happens to 7. then there will be a carry over of 2. which in turn obeys your condition of odd+even+even will not end in even . but that part is important i guess. so, which concludes the last term is going to be 2.

Sundar Raj - 7 years ago

Yeah i did the sm way... It is more easier!!

Reepa Paul - 6 years, 12 months ago

yey i did it...but i think that the last two digits could also have been 9 and 2.it would yield the same result that is last 2 digits after multiplying coul have been 52 as well

what do you want say . plz be clear in your words

akash deep - 7 years ago
Akshay Sant
Jun 3, 2014

Cooool Que bro...... @Krishna Ar it was very easy The product of last two digits with two digits doesnt matters how many digits it is... It is clear that last digit of the no. Is 2 and then after let the second last digit be p P 2 ×54 =4P+10 now as second last digit of product is 6 and greatest posiibility of coming 6 is 26 by regarding conditions thus 4P+10=26 thus P=4 There fore last two digit of 2009 digit number is 2 and 4 By order Now multiplying it By 46 We get the desired digits as 2 and 3 by order Thats it.....

COOL problem bro

@Krishna Ar

Will you please delete the question "Are you smarter than a 5th grader- Part 6· Level 4 (110 points) " from your set since I have completed all the other questions from your set but still haven't been able to make it to the leaderboard .

Thanks for the same .

Yeah, I 'll do it. But please don't post this as a solution. It is not really good to clog the discussion of this problem. Reply to me , so that I can delete your solution.

Krishna Ar - 6 years, 5 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...