Number Systems

[sukhman]

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. tens digit of n is a factor of the units digit of n .
2.the tens digit of n is 2 .
Answer A

[Brent@GMATPrepNow]

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. tens digit of n is a factor of the units digit of n .
2.the tens digit of n is 2 .
Answer A

Target question: Is n composite?

Given: n > 20

Statement 1: The tens digit of n is a factor of the units digit of n
It doesn't take long to list the 2-digit numbers that satisfy this condition:
22, 24, 26, 28
33, 36, 39
44, 48
55
66
77
88
99
All of these possible values of n are composite, so n must be composite.
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: The tens digit of n is 2.
There are several values of n that meet this condition. Here are two:
Case a: n = 21, in which case n is composite
Case b: n = 23, in which case n is not composite
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer = A

Cheers,
Brent