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Positive integer n leaves a remainder of 4 after division by

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Joined: 02 Nov 2018
Posts: 50
Re: Positive integer n leaves a remainder of 4 after division by  [#permalink]

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New post 15 Aug 2019, 12:01
Bunuel wrote:
To elaborate more.

Suppose we are told that:
Positive integer n leaves a remainder of 4 after division by 6 and a remainder of 2 after division by 8. What is the remainder that n leaves after division by 12?

The statement "positive integer n leaves a remainder of 4 after division by 6" can be expressed as: \(n=6p+4\). Thus according to this particular statement \(n\) could take the following values: 4, 10, 16, 22, 28, 34, 40, 46, 52, 58, 64, ...

The statement "positive integer n leaves a remainder of 2 after division by 8" can be expressed as: \(n=8q+2\). Thus according to this particular statement \(n\) could take the following values: 2, 10, 18, 26, 34, 42, 50, 58, 66, ...

The above two statements are both true, which means that the only valid values of \(n\) are the values which are common in both patterns. For example \(n\) can not be 16 (from first pattern) as the second formula does not give us 16 for any value of integer \(q\).

So we should derive general formula (based on both statements) that will give us only valid values of \(n\).

How can these two statement be expressed in one formula of a type \(n=kx+r\)? Where \(x\) is divisor and \(r\) is a remainder.

Divisor \(x\) would be the least common multiple of above two divisors 6 and 8, hence \(x=24\).

Remainder \(r\) would be the first common integer in above two patterns, hence \(r=10\).


Therefore general formula based on both statements is \(n=24k+10\). Thus according to this general formula valid values of \(n\) are: 10, 34, 58, ...

Now, \(n\) divided by 12 will give us the reminder of 10 (as 24k is divisible by 12).

Hope it helps.


I don't understand the bold red part. I mean I understand the LCM of 6 and 8 is 24 but I find it difficult to understand WHY/HOW this ties to the general formula. Why/how is the LCM relevant? Similarly, why is the remainder the first common integer in the above two patterns..
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Re: Positive integer n leaves a remainder of 4 after division by  [#permalink]

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New post 30 Aug 2019, 11:14
bchekuri wrote:
Positive integer n leaves a remainder of 4 after division by 6 and a remainder of 3 after division by 5. If n is greater than 30, what is the remainder that n leaves after division by 30?

A. 3
B. 12
C. 18
D. 22
E. 28


Well, this is a PS question and options are given. So let's just form the numbers.
30+3=33, 30+12=42 and so on.
33,42,48,52,58.
Now divide these numbers by both 6 and 5 and check if the remainders are satisfied.
58 works.

Hence E
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Re: Positive integer n leaves a remainder of 4 after division by   [#permalink] 30 Aug 2019, 11:14

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