Through the first pipe, the pool is filled in 10 hours, through the second in 15 hours, through the third in 30 hours.

Through the first pipe, the pool is filled in 10 hours, through the second in 15 hours, through the third in 30 hours. How many hours did the pool fill through three pipes during their joint operation.

Whole pool 1
1/10 of the pool in the first hour
1/12 of the pool per second hour
1/30 of the pool in the third hour
1/10 + 1/15 + 1/30 = 3/30 + 2/30 + 1/30 = 6/30 = 1/5 pools per hour together
1: 1/5 = 5 hours will fill the pool together