Question

Hannah can paint a room in 9 hours. Destiny can paint the same room in 15 hours. How long does it take for both Hannah and Destiny to paint the room it they are working together?

Answer 1

Amount of room hannah can paint in an hour = 1/20
amount of room destiny can paint in an hour = 1/16
number of hours = x

1/20x + 1/16x = 1
the reason why this formula is true is, the amount of the room painted in the end should be 1 (the whole room)

4/80x + 5/80x = 1
9/80x = 1
9x = 80
x = 80/9 hours
x = 8.89 hours