There pipes can independently fill a water tank in 10,12 and 15 hour.How many hours will it take for all three pipes opend together to fill half of the tank ?

A 6

B 5

C 4

D 2

Correct Answer: Option D

In 1 hour 3 pipes can fill = 1/10+1/12+1/15 = (6+5+4)/60 = 15/60 =1/4 part

So, 1/4 part fill in = 1 hour

1 part " " = 4 "

So, 1/2 part " " = 4/2 or, 2 hours (Ans.)