Set Two Q13

[wvspidermonkey]

Q13

An integer greater than 1 that is not prime is called composite. If the two-digit integer n is greater than 20, is n composite?

(1) The tens digit of n is a factor of the units digit of n.
(2) The tens digit of n is 2.

Formal Answer: D

My answer: (1) The number could be 24, 26, 28, 36, 39, etc. However, (2) only offers “the tens digit of n is 2.” The number could still be 24, 26, or 28. I don’t understand that the answer would be D when a specific number can’t be identified using both (1) and (2). Why isn’t the answer E?

[kevin]

Keep in mind that they are not asking for the actual value of n but rather whether n is composite

If the units digit u is a multliple of the tens digit t > 1, then n=10t + u, the sum of two multiples of t. Thus n is a multiple of t and is composite (not prime). (1) is sufficient

(2) not sufficient, as n could be prime (23,29) or composite

Answer A

[wvspidermonkey]

I'll go back and study this... Thank you!