Two cars leave town at the same time and travel in opposite directions. The average rate of speed of one car is 15 miles per hour faster than the average rate of speed of the other car. The cars are 315 miles apart after three hours. Find the average rate of speed of the two cars.

Let the speed of the slower car = X

The speed of the faster car would be X + 15

They both traveled 3 hours and went 315 miles

Set up an equation:

3x + 3 (x+15) = 315

Now solve for x:

3x + 3x + 45 = 315

Simplify:

6x + 45 = 315

Subtract 45 from both sides:

6x = 270

Divide both sides by 6:

x = 270/6

x = 45

The slower car was doing 45 miles per hour.

The faster car was doing 60 miles per hour.