Data Sufficiency

[email protected] wrote:If Car X followed Car Y across a certain bridge that is
1/2 mile long, how many seconds did it take Car X to
travel across the bridge?

(1) 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.
(2) Car Y traveled across the bridge at a constant
speed of 30 miles per hour.

Cant understand the first statement!

Let the time needed for car X to travel across the bridge be Tx seconds and the time for Y be Ty seconds.
We have to find Tx.

(1) 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.
Car X needs 1 second less to travel across the bridge than car Y implies Ty = Tx + 1, but we cannot find Tx from this; NOT sufficient.

(2) Car Y traveled across the bridge at a constant speed of 30 miles per hour implies 30/3600 = 1/120 miles per second.
Car Y needs Ty = 21/(1/120) = 21 * 120 seconds to travel across the bridge, but we cannot find Tx from this; NOT sufficient.

Combining (1) and (2), Ty = Tx + 1 and Ty = 21 * 120
Tx = (21 * 120) - 1; SUFFICIENT.

The correct answer is C.