Two trains leave Chicago, Illinois, traveling in the same direction along parallel tracks. The first train leaves 3 hours before the second, and its speed is 15 miles per hour faster than the second. Find the speed of each train if they are 270 miles apart 3 hours after the second train leaves Chicago.

The solution in the back of the text is "60 mph, 75 mph".

I figured that the first train's speed would be 270 miles/3 hours=90 mph . Since the second train's speed is 15 mph slower than the first train's speed, I thought the answers would be 90 mph for the first train and 75 mph for the second train. Where did I go wrong?