Sally can paint a room in 7 hours while it takes Steven 3 hours to paint the same room. How long would it take them to paint the room if the room if they worked together?
It would take __ hours
These are easy to solve when we know the rates
Sally can paint 1/7 of a room in 1 hour
Steven can paint 1/3 of a room in 1 hour
(1/7) + (1/3) = [ 7 + 3 ] / 21 = 10/21
Invert this fraction = 21 /10 = 2.1 hrs. working together