Question 822455
When the fast boat starts out, the slow boat has traveled 4.5 hours at 5 mph.
 

Since d=rt, the slow boat is d=5(4.5)=22.5 miles ahead of the fast boat when the fast boat starts out.
 
The fast boat's catch-up rate is the difference between their speeds or 20-5 or 15 mph.
  

Since t=d/r it will take the fast boat t=22.5/15 or 1.5 hours to catch up the slow boat's 22.5 miles head start.
  

Edwin