Question 981521
If Sean takes 2 hours to paint the room, how much does he paint in 1 hour?  1/2
If Frank takes 4 hours to paint the room, how much does he paint in 1 hour? 1/4
Together, they paint 3/4 of the room in one hour, or 1/4 every 20 minutes. 
4/4 would be 4times that, or 80 minutes.