Two cars start from the same place at the same time but travel in opposite directions. One car averages 42 miles per hour and the other averages 51 miles per hour. In how many hours will they be 651 miles apart

The two cars will be 651 miles apart, when the sum of the distances covered by the two cars is 651 miles.

distance = speed x time.

Let the time covered when they are 651 miles apart be x, then:

42x + 51x = 651

93x = 651

x = 651/93 = 7.

Therefore, the two cars will be 651 miles apart in 7 hours.