A Chest Of Gold To Divide Amongst Friends

A box contains gold coins.

If the coins are equally divided among six friends, four coins are left over.
If the coins are equally divided among five friends, three coins are left over.

If the box holds the smallest number of coins that meets these two conditions, how many coins are left when equally divided among seven friends?

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27 solutions

Lorenc Bushi
Jan 2, 2014

Let the number of gold coins be n n .Then,from the problem we have:

n = 6 k + 4 n=6k+4

n = 5 p + 3 n=5p+3
Doing just a litttle manipuation on both equations we get:

n = 6 ( k + 1 ) 2 n=6(k+1)-2

n = 5 ( p + 1 ) 2 n=5(p+1)-2

Logically, we realise that 6 ( k + 1 ) = 5 ( p + 1 ) 6(k+1)=5(p+1) . These numbers are divisible by 6 6 and 5 5 simultaneously.Therefore they are also divisible by 6 5 = 30 6*5=30 .So,these numbers are multiple of 30 30 .Since we want the minimum, the smallest possible result is 30 30 .Therefore 6 ( k + 1 ) = 30 6(k+1)=30 where we get k = 4 k=4 and also 5 ( p + 1 ) = 30 5(p+1)=30 where we get p = 5 p=5 .From this we get n = 28 n=\boxed{28} .When divided by 7 7 this number leaves a remainder 0 0 or 28 = 7 4 + 0 28=7*4+0 thus , the remainder is 0 \boxed{0} .

Initially, based on the given conditions - Let x be the required coins, then x = 6a+4 & x = 5b+3 - (1) thus, 6a+4 = 5b+3 or 6a-5b+1=0 Now, by trial and error method try and get the values of a and b such that the equation is satisfied hence for above equation we get values of a=4 and b=5 Putting it in the above equation (1), we get values 28 & 29. Now in order to satisfy both the conditions and to get the minimum value we take the LCM of 28 & 29 to get 812. On dividing 812 by 7, we get 0 as 812 is completely divisible by 7.

dhruv Patel - 7 years, 4 months ago

Good solution and nice explanation. Keep it up.

Prasun Biswas - 7 years, 5 months ago

why after the manipulation there are the -2 there? i don't really understand

Adeline Voon - 7 years, 5 months ago

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Adeline Voon, 6 k + 4 = 6 k + 6 2 = 6 ( k + 1 ) 2 6k+4=6k+6-2=6(k+1)-2 and so for the second equation.

Lorenc Bushi - 7 years, 5 months ago

ur awsum!!! thanks for the gr8 ans

pgk sathvikaa - 7 years, 5 months ago

nice explanation but i have a doubt. why does it have to be divisible by 30?

milind prabhu - 7 years, 5 months ago

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It has to be divisible by 30 because it is divisible by 6and 5 at the same time.and only 30and its multiples are divisible by 6&5 both. Az we need the least we chose 30.

infinity moon - 7 years, 4 months ago

Bcoz if any number which is both divisible by 5 ans 6 then this number must divisible by their product that is 5×6=30

arijit dutta - 5 years, 10 months ago

i knew the answer but broblem was of equations

Siddharth Pardeshi - 7 years, 4 months ago

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how did you get the answer without equations?

Ray Mar Pamisa - 7 years, 2 months ago

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Do manually starting from 1 up to 4 u will get n=28 mentally.if understand then reply

arijit dutta - 5 years, 10 months ago

as u know if u divide coins among 6 then 4 left this means total coins are integral multiple of 6 plus 4 thus let total coins = 6x +4 similarly when divided among 5 friends then total coins =5y+3 now by hit & trial just putx=1,2,3 so on which satisfies both equation is the answer in this case it is x=4 as we have total coins 6x+4 =5y+3

Garima Vashisht - 7 years, 3 months ago

n%5=3 so n>5+3 =8 or 5*2+3=13 but this option is not valid. Reason for that is that 13 or any other nr last ending digit as 3%2 =1 and we can see that to get n%6=4 n%2 must be =0
So the last digit of n must be 8 n%6=4 and n>6+4 from there 6+4=10+6=16+6=22+6=28
There it is smallest nr to meet conditions. and 28%7=0

Raimondas Lukosius - 3 years, 3 months ago

Great, this is an intelligent approach!! Well done!🤗👍

Prem Chebrolu - 2 years, 9 months ago

They have asked smallest.

Himanshu Parihar - 10 months, 1 week ago

I liked your approach! :)

Gaurang Pansare - 7 years, 5 months ago

hmm, very logic and realistic. keep spirit with that yow.

Hizkia Tjahyono - 7 years, 5 months ago

a good solution ... forward continues to make people want to learn

Adriansyah Adriansyah - 7 years, 4 months ago

hey I didn't understand the last part if k=4 and p=5..then how do we get n=28???????? please someone clarify this

Absaar Shoaib - 7 years, 4 months ago

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n=6k+4
n=(6x4)+4= 28 \boxed{28}

n=5p+3
n=(5x5)+3= 28 \boxed{28}

Nadya Bachtiar - 7 years, 4 months ago

Put the values of either k or p in the 1st two equation and u will get n=28.if u understand then reply

arijit dutta - 5 years, 10 months ago

I don't get it.. I came up with 58 when divided by 6 there's 4 left, and if divided by 5 there's 3 left.. so when it divided by 7 there's 6 left.. is it correct??

Ricky Honarto - 7 years, 2 months ago

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well they asked for smallest number of coins which satisfy these conditions. as 28 < 58, your answer is wrong

Shravan Jain - 7 years, 2 months ago
Kenny Lau
Jan 2, 2014
  1. List out the numbers that have remainder 4 when divided by 6: 4, 10, 16, 22, 28 , 34, 40, ...
  2. List out the numbers that have remainder 3 when divided by 5: 3, 8, 13, 18, 23, 28 , 33, ...
  3. Find out the common number between them: 28
  4. Divide that by 7 and find the remainder: 0 \boxed0

I chose this way.

Chandrika Bolloju - 7 years, 4 months ago

That's exactly how I did it! great explanation though

fatin shuhada - 7 years, 2 months ago

this was my second go but i thought first i'll write all the multiples of 7 i.e. 7,14,21,28 .... now 28 /6 gives remainder 4 and 28 /5 gives remainder 3 .

amogh joshi - 7 years, 5 months ago

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But how do you know at first that it'll be divisible by 7?

Kenny Lau - 7 years, 5 months ago

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Because it's a multiple of 7, 7 x 4 = 28

M Stevenson - 7 years, 4 months ago

Exactly how I did it. Quickest, simplest and most uncomplicated solution possible. You can do it mentally.

M Stevenson - 7 years, 4 months ago

this is the shortest way..

Ankit Soni - 7 years, 4 months ago

hmm...it seems it will take years to solve if the man must have had something like 19383743 friends

bagavath shriram - 11 months, 2 weeks ago
Pratik Vora
Dec 27, 2013

5k+3=6n+4

5k-6n=1

Obvious k=5,n=4

Total coins=28

28%7=0

How on earth is that obvious? :)

Kenny Lau - 7 years, 5 months ago

u re approach was appreciable

Ujala Jha - 7 years, 5 months ago

Excuse me? You call that a explanation? You have to think of the reader when you write the answer to a problem! For anything, you must specify how you got 4 and 5. Goodness! That makes the reader sound dumb. Do you really want that? I thought so. On a lighter not though, your equation was quite impeccable.

Jaeyun Ham - 7 years, 5 months ago

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Diofantine equations, look in the wiki.

Pablo Sanchez - 1 year, 11 months ago

Use https://brilliant.org/wiki/extended-euclidean-algorithm/ (Extended Euclidean Algorithm) to solve 5k-6n=1 .

Steve Bobs - 7 months ago
Abubakarr Yillah
Jan 2, 2014

Let the total number of gold coins be ₌ x

x ₌ 6a + 4 …..(1)

x ₌ 5b + 3 …..(2)

Equating the two gives,

6a + 4 ₌ 5b + 3

i.e 6a – 5b ₌ -1

Testing integers on the equation, the pair that gives the smallest number of gold coins in the box is 4 and 5 for a and b respectively. Thus,

x ₌ 6(4) + 4 ₌ 28.

x ₌ 5(5) + 3 ₌ 28

Therefore 28/7 ₌ 4 remainder 0 as required.

Ur method was quiet appreciable.

zarree khan - 7 years, 4 months ago

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thank you

Abubakarr Yillah - 7 years, 4 months ago
Prasun Biswas
Jan 4, 2014

Let the no. of coins be x x . Let p p and q q be arbitrary constants. From the question, we get----

x = 6 p + 4 x=6p+4 ....(i) and x = 5 q + 3 x=5q+3 ....(ii)

From (i) and (ii), we get----

6 p + 4 = 5 q + 3 6 p 5 q + 1 = 0 6p+4=5q+3 \implies 6p-5q+1=0

The equation is satisfied for the least positive values of p , q p,q with p = 4 p=4 and q = 5 q=5 . So, we get the no. of coins as x = 6 p + 4 = 6 × 4 + 4 = 24 + 4 = 28 x=6p+4=6\times 4 + 4 = 24+4 = 28

Now, if 28 coins are equally distributed among 7 people, then each will get 4 coins and 0 coins are left over. So, the answer is 0 \boxed{0}

Ashutosh Mishra
Jan 5, 2014

List a few multiples of 5 and 6 . Add 3 and 4 to them respectively. you will find that the number 28 is common in both lists( list after adding 3 and 4). When 28 is divided by 7, the remainder is zero . Therefore, answer is 0

Wtf it's hard... I don't want to waste my time. Gave some bucks 💵to do that.

SAHIL JARIAL - 1 year, 7 months ago

Be c c the number of coins. Then c 4 m o d 6 , c 3 m o d 5 c \equiv 4 \mod 6 , c \equiv 3 \mod 5 . It has got solutions for the Chinese remainder theorem , the smallest of which is 28 0 m o d 7 28 \equiv \boxed{0} \mod 7 .

Clear explanation, link provided, I like it :)

However, in English, we say "Let something be..." instead of "Be something ...", as opposed to French/Spanish/Italian. :)

Kenny Lau - 7 years, 5 months ago

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Ok , thank you :)

Alessio Di Lorenzo - 7 years, 5 months ago

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No problem :)

Kenny Lau - 7 years, 5 months ago
Ahmed Soulmani
Jul 14, 2018

We are going to proceed using The Chinese Remainder Theorem.

Let x x be the number of coins contained in the box, we have:

  • x 4 ( m o d 6 ) x\equiv 4(mod\ 6)
  • x 3 ( m o d 5 ) x\equiv 3(mod\ 5) *

First, we have x 4 ( m o d 6 ) x = 6 k + 4 x\equiv 4(mod\ 6)\ \Rightarrow \ x=6k+4 ** with k k an integer.

Substituting this result into * gives: 6 k + 4 3 ( m o d 5 ) 6k+4\equiv 3(mod\ 5) hence 6 k 1 ( m o d 5 ) 6k\equiv -1(mod\ 5) .

Now, what we should try to do here is to get rid of that 6 6 , to have an expression which only contains k k . To do that, we should find the modular multiplicative inverse of 6 6 modulo 5 5 . We know that 6 1 ( m o d 5 ) 6\equiv 1(mod\ 5) which means that 1 1 is the modular multiplicative inverse of 6 6 modulo 5 5 .

In cases of bigger numbers, one should use the euclidean algorithm to determine the modular multiplicative inverse.

Hence 6 × 1 × k 1 × 1 ( m o d 5 ) k 1 ( m o d 5 ) 6\times 1\times k\equiv -1\times 1(mod\ 5)\Rightarrow k\equiv -1(mod\ 5) , which means k k can be written as: k = 5 i 1 k=5i-1 with i i an integer.

Substituting this result into ** gives: x = 6 ( 5 i 4 ) + 4 x=6(5i-4)+4 which means x = 30 i 2 x=30i-2

Since x > 0 x>0 , the minimum value x x can take is 28 \boxed{28} , attained with i = 1 i=1

So the minimum number of coins that satisfies the two desired conditions is 28 28 .

And we know that 28 0 ( m o d 7 ) 28\equiv 0(mod\ 7) , which means that, if we equally divide these coins among 7 people, no coins will be left.

Hence, the result is 0 \boxed{0} .

Mohamed Mokhtar
Dec 3, 2020

x = 4 (mod 6) => a1 (mod m1)

x = 3 (mod 5) => a2 (mod m2)

by the Chinese remainder theorem , we know there exists a solution in the form :

x = a1 * m2 *(the multiplicative inverse of m2) + a2 * m1 * (the multiplicative inverse of m1)

the multiplicative inverse of 5 (mod 6) = 5 and the multiplicative inverse of 6 (mod 5) = 1

x = 4 * 5 * 4 + 3 * 6 * 1 = 118 = 28 (mod 30) which is the smallest amount of coins that meet the conditions, the solution then is obvious there is no coin left over

Raj Thaker
Mar 6, 2014

There is an easy way. You know that there is 3 coins left after divided by 5. Therefore number has to end with either 3 or 8 due to the fact that multiples of 5 end with 0 and 5. After that you go from 13 to 18 to 23 to 28 and oh that works!

Let N be the number of gold coins where

  • N ( m o d 6 ) 4 N \pmod{6} \equiv 4
  • N ( m o d 5 ) 3 N \pmod{5} \equiv 3

By trial-and-error, If N = 6 N = 6 , 6 ( m o d 6 ) 0 6 \pmod{6} \equiv 0

So, I add 4 to 6 to make N N follow the first premise. If N = 10 N = 10 , 10 ( m o d 6 ) 4 10 \pmod{6} \equiv 4 and 10 ( m o d 5 ) 0 10 \pmod{5} \equiv 0

Continue adding 6 to N N until you find the value of N N that will follow both premises. If N = 16 N = 16 , 16 ( m o d 6 ) 4 16 \pmod{6} \equiv 4 and 16 ( m o d 6 ) 1 16 \pmod{6} \equiv 1

If N = 22 N = 22 , 22 ( m o d 6 ) 4 22 \pmod{6} \equiv 4 and 22 ( m o d 5 ) 2 22 \pmod{5} \equiv 2

If N = 28 N = 28 , 28 ( m o d 6 ) 4 28 \pmod{6} \equiv 4 and 28 ( m o d 5 ) 3 28 \pmod{5} \equiv 3

Therefore, there are 28 gold coins.

When 28 coins are to be equally divided among 7 people, there are 28 ( m o d 7 ) 0 28 \pmod{7} \equiv 0 gold coins left over.

Kevin Fan
Dec 22, 2019

You can easily apply the Chinese Remainder Theorem. Set up the two modular functions x = 3 m o d ( 5 ) x= 3 \mod (5) and x = 4 m o d ( 6 ) x=4\mod(6) .

Next, let x = 6 j + 4 x=6j+4 .

Thus, 6 j + 4 = 3 m o d ( 5 ) 6j+4=3\mod(5) . From here, solve for j j .

j = 4 m o d ( 5 ) j=4\mod(5) .

Then, let j = 5 k + 4 j=5k+4 , as 5 k + 4 = 4 m o d ( 5 ) 5k+4= 4\mod(5) is clearly true.

Substitute j j with 5 k + 4 5k+4 in x = 6 j + 4 x=6j+4 , so x = 30 k + 28 x=30k+28 .

That means that x = 28 x=28 . You can check that this is true easily.

28 % 7 = 0 28\%7=0 , so the final answer is 0 \boxed{0}

Dustin Watson
Mar 1, 2018

First off, crt dependents on relative Primes. 6 isn't Prime. 6 is equal to 2 * 3. Both prime. Rewrite problem as 2 congruent to x mod 3 because 2 sets of 2 is 4.

Now all mods are prime, so 7 5 3 = 105

Next substitute the mod in the method in the lesson. Get 7(10k + 5) or 70L + 35

35 is the number of interest, and 35 is a factor of 105, so no remainder. (105 = 3 35, 35 = 7 5)

It depends on the moduli being coprime. ie their GCF is 1. 5 and 6 are coprime, thus the CRT can be applied.

Insert Name - 4 months ago

Use the chinese remainder theorem

Olaleye Hammed
Apr 15, 2017

Let the number of coins be x.from the question we can have, X=6j +4 and j=5k +4 When substitute into the first equation, we get: X=30k+28 , if k=0 , then we have: X=28 and 7 divided by this number is 4 remainder 0 So,remainder equal 0

Sree Suresh
Mar 15, 2014

based on the given data, 6x+4=n; 5y+3=n; hence 6x-5y+1=0; to satisfy this equation put x=4 and y=5. so n=28. if its distributed among 7 reaming is 0.

According to the information, we get the two equations-5y+3=x and 6z+4=x where x is the no. of gold coins and z a y are two real numbers(I got the equations by divisor-remainder theorem).Solving the equations by trial and error method i got x=28.Now if we divide 28 by 7, naturally the remainder will be 0.

Sabyasachi Halder
Jan 24, 2014

Let the number of coins = n. Then n = 6x+4 and n = 5y + 3. Where x & y are positive integer. So 6x + 4 = 5y + 3 or, 6x - 5y + 1 = 0, (x, y) = ( 5k - 1, 6k - 1) where k is integer. For k = 1, (x, y) = (4, 5). So smallest n is 28. 0 coin left when equally divided among seven friends?

Nick De Ruyck
Jan 21, 2014

So divided by 6 is 4 left: The possible amount of coins are: 4, 10, 16, 22, 28, 34, ... (4+6n) So divided by 5 is 3 left: The possible amount of coins are: 3, 8, 13, 18, 23, 28, 33, ... (3+5m) The smallest number that we get in both lists is 28. Now we divide 28 by 7: 28/7=4 with no rest. Therefor the solution is 0.

Motiur Rahman
Jan 18, 2014

Total gold coins 28 If the coins are equally divided among six friends, four coins are left over it is 4 6=24+4=28 If the coins are equally divided among five friends, three coins are left over it is 5 5=25+3=28 If the coins are equally divided among seven friends, zero coins are left over it is 7*4=28+0=28 That's all

Indu Mathi
Jan 15, 2014

Now let the coins in the box was 28..... When 6 friends equally divided there 4 coins left...(28%6=4) When 5 friends divided there 3 coins left....(28%5=3) When 7 friends divided there 0 coins will left...(28%7=0)

haha

Baalu Durgadas - 7 years, 4 months ago
  1. (the smallest possible number of coins) divided by 6 = some number plus 4
  2. (the smallest possible number of coins) divided by 5 = some number plus 3

Starting with requirement 2, any number that is divisible by 5 will end in a 5 or a 0, so any number that, when divided by 5, gives a remainder of 3, will end in a 3 or 8. Therefore we know that the number of coins will end in either a 3 or 8.

As for requirement 1, any number that is divisible by 6 will be even, and any number that when divided by 6 gives a remainder of 4 will be even. The number of coins must then be even.

Requirement 1 shows that the number of coins must end in a 3 or 8, and requirement 2 shows that the number of coins must be even, therefore the number of coins cannot end in a 3, it must end in an 8.

8 and 18 do not follow the requirements, but 28 does. 28 divided by 7 gives 4 with no remainder, so the solution to this problem is 0.

Krati Gupta
Jan 7, 2014

Let the smallest no. of gold coins in the box be x According to question, when x is divided by 6, 4 is the remainder. That means, ( x +2) is divisible by 6 According to question, when x is divided by 5, 3 is the remainder. That means, ( x +2) is divisible by 5 Hence, ( x +2) is the L.C.M. of 5 and 6 i.e. 30 So, x = 28 Now, when 28 is divided by 7, the remainder is 0. Therefore, no. of coins left when divided among 7 friends = 0

Very good application of LCM.

Manohar Mathur - 7 years, 4 months ago
Rajat Jain
Jan 6, 2014

here no of coins can be written as, '6a+4 = 5b+3' = no of gold coins by putting different values of 'a', we can find the minimum value which is ''28''

Reeshabh Ranjan
Jan 6, 2014

I am using a very simple method Let no. of coins be x Acc. to the q., if we remove 4 and 6 from x respectively for the division by 6, the division will be complete. So:- so (x-4)/6 ϵ N also, (x-3)/5 ϵN

Using Hit and Trial, we get 28 to fulfill this condition being the smallest possible Natural Number [No. of coins can't be in decimals or negative].

So 28/7, gives 0 as the remainder :D .

Lakshmi Tumati
Jan 2, 2014

In the problem, it says when the coins are equally divided among six people. four coins are left over, That means that the number of gold coins is four below or four above a multiple of six. Then, it says when the coins are equally divided among five people three coins are left over. That means that the number of coins is three below or three above a multiple of five. The smallest number meeting the two conditions is 28.

Hùng Minh
Jan 2, 2014

Let y is the total gold coins. So y = 6x + 4 and y = 5z + 3. We note z > x, x, y, z is positive intergers and 6x + 4 = 5z + 3 <=> x = (5z - 1)/6. We try with z = 1, 2, 3, 4, 5.... And the smallest value of z is 5 and x is 4 <=> y = 28 <=> 28/7 = 4 and the remainder is 0.

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