Question

A fire hose can fill a certain tank with water during 1 hour. A 2nd fire hose can fill the same tank in half hour. A 3rd fire hose can fill the same tank in quarter hour.
find the time necessary to fill the same tank by the 3 fire hoses together

Answer 1

1/T = 1/1 + 2/1 + 4/1 = 12,
T = (1/12)h = 5 Min.

Answer 2

Correction:

!/T = 1/1 + 2/1 + 4/1 = 7,
T = (1/7) hr. = 8.57 Min.