Two Pipes A and B can fill a tank in 6 hours and 4 hours respectively; If they are opened on alternative hours and if pipe A is opened first, in how many hours, the tank shall be full ?

A 4

B 4.5

C 5

D 5.5

Correct Answer: Option C

Solution:

(A+B) can fill in first (1+1) = 2 hours 1/6 + 1/4 = (2+3)/12 = 5/12 part.

(A+B) can fill next 2 hours = 5/12 part.

In first 4 hours the tank fill = 5/12 + 5/12 = 10/12 = 5/6 part.

Remaining part 1- 5/6 = 1/6 part.

In 5th hour A can fill 1/6 part, So total time 4+1 = 5 hours.