A driver noticed another car rapidly approaching from behind while driving at 72 mph with cruise control on. When the other car passed, the driver counted seconds until they both passed the same bridge in the distance. By counting 11 seconds for the other car and 12 seconds for themselves, the driver determined the other car was traveling at approximately 78.5 mph.

1. I was driving on Interstate 90 and noticed in my rear view mirror a car that was rapidly
approaching. I look at my speedometer and saw that I was going 72 miles per hour (I had my
cruise control on). When the fast car passed me, I looked at my watch and began to count the
number of seconds that had passed. I saw a bridge in the distance. When I reached 11 seconds,
the fast car drove past the bridge. I continued to count and at 12 seconds, I drove past the same
bridge.
How fast (miles per hour) was the other car going?
Solution
v*11=72*12
==> v=78.545 mi/hr