Question

An earthquake strikes and an isolated area is without food or water. three crews arrive. one can dispense needed supplies in 4545 ​hours, a second in 9090 ​hours, and a third in 6060 hours. how long will it take all three crews working together to dispense food and​ water?

Answer 1

It'll take 10 hours for the crew to finish the work.

1 person takes 20h, the second would hake 30h, third 60h.
1/20+1/30+1/60=1/10