Official Solution:If \(m\) and \(n\) are integers, is \(\frac{10^m + n}{3}\) an integer? Essentially, the question asks whether \(10^m+n\) is divisible by 3. For \(10^m+n\) to be divisible by 3:
A. It must be an integer, and B. the sum of its digits must be a multiple of 3.
(1) n = 5. If \(m < 0\) (-1, -2, ...), then \(10^m+n\) will not be an integer at all (for example, if \(m=-1\), then \(10^m+n=\frac{1}{10}+5=\frac{51}{10}\neq{integer}\)), thus it won't be divisible by 3. But if \(m \geq{0} \) (0, 1, 2, ...), then \(10^m+n\) will be an integer, and the sum of its digits will also be divisible by 3 (for example, for \(m=1\), then \(10^m+n=10+5=15\), which is divisible by 3). Not sufficient.
(2) \(mn\) is an even number.
Clearly insufficient, as \(m\) can be -2 and \(n\) any integer, and the answer to the question will be NO, or \(m\) can be 0 and \(n\) can be 2, and the answer to the question will be YES. Not sufficient.
(1)+(2) From \(mn=even\) and \(n=5\), it's still possible for \(m\) to be a negative even integer (-2, -4, ...), so \(10^m+n\) may or may not be divisible by 3. Not sufficient.
Answer: E