Answer to GMAT Practice Question #4

The following is the answer to GMAT Practice Question #4

Answer: A

Explanation:

The question is asking whether or not k is a prime number.

The first statement tells us that the units digit is a multiple of the tens digit. What does that mean? Think through it – each two digit number has a units digit and a tens digit, and a multiple of a particular number can be divided by that number without a remainder.

Let’s put some numbers in to see what that would look like.

For example, k could be 36. The units digit (6) is a multiple of the tens digit (3). k could not be 35, because the units digit (5) is not a multiple of the tens digit (3).

Once we understand that, the problem is more straightforward. There are actually not too many options for what k could be, so we can just start with k having a tens digit of 2 and working up. The following numbers could be k:

20, 22, 24, 26, 28
30, 33, 36, 39
40, 44, 48
50, 55
60, 66
70, 77
80, 88
90, 99

Since none of these numbers are prime, statement 1 is enough information.

Statement 2 tells us that the units digit of k is 3. There are certainly two-digit prime numbers that end in 3, such as 23 or 53. But there are also two-digit numbers that end in 3 that are not prime, such as 33 and 63. So statement 2 is not enough information.

The answer and explanation to GMAT Practice Question #4 was written by Integrated Learning, a company that provides professional and private one on one tutoring services.