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Re: If p and q are different prime numbers, m and n are integers, and pq [#permalink]
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gmatophobia wrote:
yrozenblum wrote:
If p and q are different prime numbers, m and n are integers, and pq is a divisor of mp + nq, which of the following must be true?

I. p is a divisor of n.

II. pq is a divisor of mp.

III. is a divisor of mn.

A) I only
B) II only
C) III only
D) I and II
E) I and III


pq is a divisor of mp + nq

\(\frac{mp + nq }{ pq}\) = integer

\(\frac{mp }{ pq} + \frac{nq }{ pq}\) = integer

\(\frac{m }{ q} + \frac{n }{ p}\) = integer

As q and p are different prime numbers, the above statement is possible when \(\frac{m }{ q}\) and \(\frac{n }{ p}\) are integers.

I. p is a divisor of n.

Inference: \(\frac{n}{p}\) = integer

This is correct. For the relationship mentioned in the question to hold true, \(\frac{n}{p}\) must be an integer. We can eliminate B and C.

II. pq is a divisor of mp.

Inference: \(\frac{mp}{pq} = \frac{m}{q} \) = integer

This is correct. For the relationship mentioned in the question to hold true, \(\frac{m}{q}\) must be an integer.

We don't have to check for III as none of the options have all three correct.

Option D


gmatophobia - can you explain the highlighted part in bit more detail specifically addressing why can't m/q and n/p be fractions and their total is an integer?

Thanks a lot
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If p and q are different prime numbers, m and n are integers, and pq [#permalink]
Sonia2023 wrote:
why can't m/q and n/p be fractions and their total is an integer?

­
\(\frac{mp + nq }{ pq} = k\) in which \(k\) is an integer.­

\(mp + nq = kpq\)

\(mp = kpq - nq\)

\(m = kq - \frac{nq}{p}\) in which \(m\) and \(kq\) are an integers, so ­\(\frac{nq}{p}\) must be an integer.

as long as ­\(\frac{nq}{p}\) is an integer, ­\(\frac{n}{p}\) must be an integer because \(p\) and \(q\) are coprime.­
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If p and q are different prime numbers, m and n are integers, and pq [#permalink]
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