A mason can build a tank in 12 h. After working for 6 h. he took the help of a boy and finished the work in another 5 h. The time that the boy will take alone to complete the work is?

Mason's 1 h work = 1/12
Mason's 6 h work = 6/12 = 1/2
Remaining work = 1 - (1/2) = 1/2
Remaining work can be finish in 5 h total work can be finished in 2 x 5 = 10 h.
(1/12) + (1/B) = (1/10)
⇒ 1/B = (1/10) - (1/12) = 1/60
Boy can complete the work in 60 h.