Shannon can paint a room in 16 hours. Emily can paint the same room in 20 hours. How long does it take for both Shannon and Emily to paint the room it they are working together?
c is the combined time
Shannon paints c/16, and Emily paints c/20
the two fractions add to 1 (the whole room)
c/16 + c/20 = 1 ... 5c + 4c = 80