Question 822455
Catch up speed = 20-5 = 15 mph 
 slow boat speed = 5mph. In 4.5 hours it travels 5*4.5 = 22.5 miles 
 when fast boat starts catch up distance = 22.5 miles
 catch up speed = 15 mph 
 catch up time = 22.5/15 
 = 1.5 hours