Last Updated on May 6, 2023

GMAT OFFICIAL GUIDE DS

Solution:

We are given that Car X follows Car Y across a bridge that is ½ mile long. We need to determine the time, in seconds, it took Car X to travel the ½ mile across the bridge.

Statement One Alone:

Car X drove onto the bridge exactly 3 seconds after Car Y drove onto the bridge and drove off the bridge exactly 2 seconds after Car Y drove off the bridge.

From statement one, we know that Car X gained 1 second on Car Y while the two cars traveled over the bridge. Thus, Car X took 1 second less than Car Y took to cross the bridge. However, without knowing the actual time it took Car Y to cross the bridge, we still don’t know the number of seconds it took Car X to cross it. Statement one alone is insufficient to answer the question. We can eliminate answer choices A and D.

Statement Two Alone:

Car Y traveled across the bridge at a constant speed of 30 miles per hour.

From statement two we can determine the time it took Car Y to cross the bridge.

time = distance/rate

time = (1/2)/30

time = 1/60 hour

Since 1 hour = 3,600 seconds, 1/60 hour = 3,600 x 1/60 = 60 seconds.

We know that Car Y took 60 seconds to travel across the bridge. However, we do not know anything about Car X, so statement two is insufficient to answer the question. We can eliminate answer choice B.

Statements One and Two Together:
Because we know that that Car Y took 60 seconds to travel across the bridge and Car X took 1 second less, we know that it took Car X 59 seconds to travel across the bridge.

Answer: C