Interchanging quotients and divisors

Are there positive integers ( a , b ) (a, b) such that

  • a a divided by b b gives remainder 6, and
  • b b divided by a a gives remainder 5?
Yes, there are infinitely many solutions Yes, there are finitely many solutions No, there is no solution

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8 solutions

roses are red, violets are blue, putting a=6 and b=(6*n)-1 where n∈N ∩ n>1 would be the right clue.

Funny poem, i did not know before...

Leonblum Iznotded - 2 years, 11 months ago

Math is beautiful, as always!

Mykyta Shvets - 2 years, 11 months ago

Wow so poetic

Laura Gao - 2 years, 11 months ago

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Its more annoying now. In the first question of the week he used the same "poetic" language

James Bacon - 2 years, 11 months ago

what does "n∈N ∩ n>1" mean?

Laura Gao - 2 years, 11 months ago

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It means that "n is a Natural number and n > 1"

Mykyta Shvets - 2 years, 11 months ago

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Ok I get it now.. Thank you

Laura Gao - 2 years, 11 months ago

I think this means that n belongs to the Natural numbers and at the same time n has to be higher than 1. Not sure

Ferran Marquez - 2 years, 9 months ago

Guys like you make my day, adding humour to irrelevant places

Yash Bahl - 2 years, 11 months ago

This should NOT be in the "Basic" category! It should be in the "Intermediate" category.

Dennis Rodman - 2 years, 8 months ago
X X
Jul 1, 2018

If a > b a>b ,then b = 5 b=5 ,but a a divided by 5 5 gives a remainder 6 6 ,which is impossible.

If a < b a<b ,then a = 6 a=6 ,so b b divided by 6 6 gives a remainder 5 5 , we have b = 11 , 17 , 23... b=11,17,23...

Hence there exists a family of solutions ( a , b ) = ( 6 , 6 n + 5 ) (a,b)=(6,6n+5) ,where n n is a positive integer.

Are there other families of solutions, or is this the only one?

Calvin Lin Staff - 2 years, 11 months ago

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I think this is the only one,because a a can only be 6

X X - 2 years, 11 months ago

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Yes, this is indeed all of the solutions.
How can we show that?

Calvin Lin Staff - 2 years, 11 months ago

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@Calvin Lin I editted the solution to explain it.

X X - 2 years, 11 months ago

a can only be 6 because otherwise if you want any other number: in order to fulfill the first statement you have to take two numbers a>b (otherwise if b>a, a=6 as any other number would give quotient 0 and remainder= to the number itself, which would be any number other than 6) e.g. a=20, b=7

now, since a>b, this statement only works for b=5 (see explanation in brackets above) but if b= 5, the first statement would be invalidated because the remainder must always be less than the divisor and in the first case, we need a remainder of 6 which is not possible as 6>5

so the only family of numbers for which this works is a=6, b= 6n+5, n is a natural number

hence proved

Keertana Pillai - 2 years, 11 months ago

This very interesting i like your solution

Andrew Cook - 2 years, 11 months ago

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Thank you!

X X - 2 years, 11 months ago

I apologise for not being very smart (so I am 100% sure you are 100% correct) but I don’t understand this...

I thought we were meant to find solutions where a and b solved both of the statements.

a = 6 b = 11,17... work for second the statement, but substitute those into the first statement and it’s no go.

You can’t solve both statements with a = 6, b = 11 so why isn’t the answer “No, there is no solution”

Michael Leslie Troth - 2 years, 11 months ago

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6 divided by 11 leaves a remainder 6

X X - 2 years, 11 months ago

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Of course it does! Sorry.

Michael Leslie Troth - 2 years, 11 months ago

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@Michael Leslie Troth Don't apologize for learning, you are asking the right questions!

Robert H - 2 years, 11 months ago

@Michael Leslie Troth Also, your question has not been answered sufficiently. You got hung up the same way I did by confusing "remainder" with "result."

If we assume that we are looking for a number set that results in fractions , then there is no solution. If we assume we are looking for a number set that results in remainders , then this is possible with modular arithmetic!

i.e.

We can't solve with arithmetic We can solve with modulo arithmetic !

Here's the shorthand result: https://www.omnicalculator.com/math/modulo?c=USD&v=x:11,y:6

Here's the Brilliant.org course on Modulo Arithmetic: https://brilliant.org/wiki/modular-arithmetic/

Have at it.

Best,

  • Rob

Robert H - 2 years, 11 months ago

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@Robert H Thank you, Rob, I was confused on the modulo arithmetic. I appreciate the tips.

Chris Lee - 2 years, 11 months ago

You could do a better job of explaining that the remainder is only 6 when performing modulo arithmetic. A lot of new learners are unfamiliar with number theory and will continue to struggle unnecessarily if they aren't pointed in the right direction.

Robert H - 2 years, 11 months ago

Agree with you.

Gabby Romano - 2 years, 11 months ago

this is smart

Laura Gao - 2 years, 11 months ago

6 = 11 x (-1) + 5 6 = 17 x (?) + 5 ... in what sense is the remainder of a=6 divided by b=17 equal to 5?

Joao Neves - 2 years, 11 months ago

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Oh I think you mixed up the conditions. a divided by b must have a remainder of 6, which 6/17 does satisfy.

Peter Chang - 2 years, 11 months ago
Miguel B
Jul 9, 2018

We have a solution if a = b × n + 6 a=b\times n+6 and b = a × m + 5 b=a\times m+5 , for integer quotients n , m n,m . Replacing one equation on the other one gets that a ( 1 n m ) = 5 n + 6 b ( 1 n m ) = 6 m + 5 a(1-nm) = 5n+6 \\ b(1-nm) = 6m+5 If n m = 0 nm=0 , then we have positive integer solutions given by a = 5 n + 6 b = 6 m + 5. a = 5n+6\\ b= 6m+5. This is the case if either n n or m m are zero. Let's see what happens in each case:

  • If n = 0 n=0 then a = 6 a=6 and b = 6 m + 5 b=6m+5 for any m m and therefore there are infinite solutions.
  • If m = 0 m=0 then b = 5 b=5 which means that the remainder of a / b a/b is less than 5 and therefore cannot be 6. So this one case is not a solution.

We conclude that there are infinite solutions given by a = 6 a=6 and b = 6 m + 5 b=6m+5 for m = 1 , 2 , m=1,2,\dots .

But m = 0 m = 0 , for example, does not work. So, the conclusion,

there are infinite solutions

can only follow after making sure that n = 0 n = 0 leads to an infinite family of solutions (as you did).

Алексей Ковалёв - 2 years, 11 months ago

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You are right! I jumped too quickly to a wrong conclusion. I've rewritten the answer to reflect this.

Miguel B - 2 years, 10 months ago

Exactly the way I did it

Emily Zhang - 2 years, 11 months ago

6 / 11 6/11 has a remainder of 6 6 and 11 / 6 11/6 has a remainder of 5 5 .

a = 6 a = 6 and b = 6 n + 5 b = 6n+5 will always follow the given relationship for n Z + n \in \mathbb{Z}^{+}

Thus, there are infinitely many pairs ( a , b ) (a, b) .

p.s. Sorry, for the error before.

Are you sure? For example if n = 2 n = 2 , we get a = 12 , b = 22 a = 12, b = 22 . What is the remainder when 22 is divided by 12?

Calvin Lin Staff - 2 years, 11 months ago
Utkarsh Duvey
Jul 10, 2018

Write , a = bq + 6 \textbf{a = bq + 6} and b = ap + 5 \textbf{b = ap + 5} . Subtracting we get a-b = bq - ap + 1 \textbf{a-b = bq - ap + 1} which can be written as ax + by = 1 \textbf{ax + by = 1} .where x,y,p,q are integers. Now from Bezout Identity there exist solution if gcd(a,b) = 1 \textbf{ gcd(a,b) = 1} .Since there are infinite primes which can choose any 2.So there exist infinite solution.

I'm not quite sure what you're saying.

You start off assuming that solutions exist, and conclude that gcd ( a , b ) = 1 \gcd(a,b) = 1 is a necessary condition (which I agree with). However, how does that prove that there are "infinite primes which can choose any 2. so there exist infinite solution"?

In particular, are you saying that "Let's fix x x and y y to be primes, and find any a , b a, b that satisfy a x + b y = 1 ax + by = 1 , then they will satisfy the conditions of the question?" In particular, note that gcd ( a , b ) = 1 \gcd(a,b) = 1 is not a sufficient condition to find a solution to the problem.

Calvin Lin Staff - 2 years, 11 months ago
Ajay Sharma
Jul 13, 2018

A=NB+6 where n>=0.
B=MA+5 where m>=0 combining both A(1-M)=(N-1)B+11 Now m can only be 0 or 1 For M =0 N have infinite solutions , similarly for 1

Jason Nicholl
Jul 11, 2018

A good way to try and solve problems like these is to start with the smallest possible solution and work your way up.

The smallest value that can have remainder of 5 is 5. Similarly, the smallest value that can have a remainder of 6 is 6.

If we try substituting a a with 6 and b b with 5, it doesn't work because 6 divided by 5 gives a remainder of 1.

No matter how many times we add 5 to a a , we will never get anything but a remainder of 1, since 5 goes into 6 once with a remainder of 1. Therefore, in order to have a remainder of 6 for a a divided by b b , b b must be greater than a a .

5 divided by 6 already has a remainder of 5, so we don't want to add anything extra. In this case, we should add 6, since 6 divides into 6 perfectly. It will make b b larger without adding anything to the remainder.

If we add 6 to b b , we get 11.

( 6 , 11 ) (6,11) satisfies both conditions. Furthermore, we can keep adding 6 to b b to get the same result, thus meaning there are infinite solutions.

Very clear and detailed solution. Thanks for sharing a solution without using any equations.

Pi Han Goh - 2 years, 11 months ago
Rami 120
Jul 10, 2018

The solution is that you have to choose b = one of the multiples of 6 then substract that number by 1 ,then divide it by a = 6 and you'll have the remainder equals to 5, then the next step is to divide a = 6 by the number b you have choosen (the numbers are infinite but have to meet the first condition) then you'll get the result 0 ( because you can't divide a small number = 6, by a big number = the number you have chosen ) and the remainder equals to 6.

yes, i think substracting 1 thing is only for obtaining valid values of b.after that say b=11, divide a=6 by b=11, you get remainder of 5. and yes smaller numbers can be divided by bigger numbers and still leave remainders.

drought RWS&S - 2 years, 11 months ago

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sorry for the error above, divide a=6 by b=11 leaves a remainder of 6, my holy.

drought RWS&S - 2 years, 11 months ago

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