Bunuel
Are positive integers m and n both multiples of 3?
(1) Two-digit number mn, where m is the tens digit and n is the units digit, is a multiple of 9
(2) n is a multiple of 3
M36-129
Official Solution:Are positive integers \(m\) and \(n\) both multiples of 3? (1) Two-digit number \(mn\), where \(m\) is the tens digit and \(n\) is the units digit, is a multiple of 9
An integer is a multiple of 9 if the sum of its digit is a multiple of 9. So, \(m+n\) must be a multiple of 9.
If two-digit number \(mn\) is 36, 63 or 99, then the answer is YES;
If two-digit number \(mn\) is 18, 27, 45, 54, 72 or 81, then the answer is NO.
(2) \(n\) is a multiple of 3
No info about \(m\). Not sufficient.
(1)+(2) If \(m\) is not a multiple of 3, then \(m+n=(not \ a \ multiple \ of \ 3) + (a \ multiple \ of \ 3) = (not \ a \ multiple \ of \ 3) \) but we know that \(m+n\) is a multiple of 9, so it's also must be a multiple of 3. Thus, \(m\) must also be a multiple of 3. Sufficient.
Answer: C