How long will it take train b to catch train a?

Bob Thomas: How long will it take train b to catch train a?
Train a and train b travel down parellel tracks. Train a travels 100mph and passes the station at 9:20pm. Train b travels 120mph and passes the station at 9:35pm. how long will it take for train b to catch train a?

Answers and Views:

Answer by shhrewd
Train A has a 15 minute headstart on train B.
Rate * time = distance
100mph * 1/4 hour = 25 miles

Train A has a 25 mile headstart on train B.
Difference between the trains rates = 20 mph
Rate * time = distance
20mph * x hours = 25 miles (notice how train b is catching up 20miles every hour)
x = 5/4

Train B will catch train A in 75 minutes or 5/4 hours.