On her way home from work, Janet drives through several

We are given that Janet drives through several tollbooths on her way home from work. We need to determine whether there is a pair of tollbooths that are less than 10 miles apart.

Statement One Alone:

The first tollbooth and the last tollbooth are 25 miles apart.

Since we do not know the number of tollbooths between her home and her work, we cannot determine whether there is a pair of these tollbooths that are less than 10 miles apart.

For example, if there are only two tollbooths, then they are 25 miles apart and thus greater than 10 miles apart. However, if there are three tollbooths, there could be a pair of tollbooths that are less than 10 miles apart. Statement one alone is not sufficient. We can eliminate answer choice A.

Statement Two Alone:

Janet drives through 4 tollbooths on her way home from work.

Since we do not know the distance between her home and her work, we cannot determine whether there is a pair of tollbooths that are less than 10 miles apart. Statement two alone is not sufficient. We can eliminate answer choice B.

Statements One and Two Together:

From statements one and two, we know that the distance between the 1st and 4th tollbooths is 25 miles, which means on average, approximately 8 miles are between each consecutive tollbooth. For example, it’s possible that the distances between the 1st and 2nd tollbooths, 2nd and 3rd tollbooths, and 3rd and 4th tollbooths are, respectively, 8 miles, 8 miles, and 9 miles. In this case, we have a pair (actually three pairs) of tollbooths that are less than 10 miles apart. Furthermore, regardless of how we adjust the number of miles for the three distances, there must be at least one pair of tollbooths less than 10 miles apart. For example, let’s say the distance between the 1st and 2nd tollbooths was increased to 10 miles and the distance between the 2nd and 3rd tollbooths was increased to 12 miles; then, the distance between the 3rd and 4th tollbooths would have to be 3 miles, which is less than 10 miles. Thus, the two statements together are sufficient.

Answer: C