Rational Expression Word Problem

Question

Deron can paint a room in 3 hours working alone. Tristan can paint the same room in 6 hours working alone. How long after Deron starts to paint the room will it be finished if Tristan joins him 1 hour later?

Answer 1

Deron paints at the rate of 1/3 room per hour

Tristan paints the room at 1/6 room per hour. 

In the first hour, Deron has painted a third of the room already leaving 2/3 room remaining. And when they begin to paint together, their rates add together so that they're painting at 1/3+1/6=1/2 room per hour. 2/3 room/ (1/2 room/hour) comes to be 4/3+1 or 7/3 an hour=2 hours and 20 minutes.