Hannah can paint a room in 9 hours. Destiny can paint the same room in 12 hours. How long does it take for both Hannah and Destiny to paint the room it they are working together?

Answer 1

Rates add, Hannah's rate is 1 room / 9 hrs. Destiny's rate is 1 room / 12 hours.

The combined rate is therefore 1/9 + 1/12 = 7/36.

Time for one room is thus 1 / ( 7/36) = 36/7 hours.