1.  Two pipes A and B together can fill a cistern in 4 hours. Had they been opened separately, then B would have taken 6 hours more than A to fill the cistern. How much time will be taken by A to fill the cistern separately?
A. 1 hour
B. 2 hour
C. 6 hour
D. 8 hour

Answer: Option C

Explanation:

Let the cistern be filled by pipe A alone in x hours.

Then, pipe B will fill it in (x + 6) hours.

1 + 1 = 1
x (x + 6) 4

x + 6 + x = 1
x(x + 6) 4

x2 - 2x - 24 = 0

(x -6)(x + 4) = 0

x = 6.     [neglecting the negative value of x]