Two cars start at the same time, but travel in opposite directions. one car's average speed is 30 miles per hour (mph). at the end of 4 hours, the two cars are 400 miles apart. find the average speed in mph of the other car.

30 (miles) times 4 (hours) equals to 120 miles. 400 miles minus 120 miles equals 280 miles (distance the other car has driven). 280 divided by 4 (because we want to find the speed for only one hour) equals 70 miles.
The average speed of the other car is 70 mph