Hannah can paint a room in 20 hours. destiny can paint the same room in 16 hours. how long does it take for both hannah and destiny to paint the room it they are working together.

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

Horizontal change or the change of x