A car is traveling on a highway at a constant speed of 55mph. Another car is behind this car and is gaining 1/2 mile on the first car every 15 minutes. How fast is the car travelling?

For every 15 minutes the second  gained 1/2 mile.

for an Hour it gains 4*1/2 =2 miels an hour.

The speed of the second car =55+2=57 mph.