Alex gets to work by biking to the train station, riding the train, then walking. His entire trip to work is 15 miles. If he bikes 1/3 of the way and walks 1/5 of the way, how many miles did he travel on the train?

Answer 1

1/5 of 15 miles is 3 miles

Answer 2

bike+walk = 1/3 + 1/5 = 8/15
the remaining 7/15 was done by train.

7/15 * 15 = 7 miles by train

Or, you can say
1/3 * 15 = 5 miles biked
1/5 * 15 = 3 miles walked
15-8 = 7 miles by train