Question

Pipes A and B can fill a cistern in 15 hours together. But if these pipes operate separately A takes 40 hours less than B to fill the tank. In how many hours the pipe A will fill the cistern working alone?

• A. 60
• B. 20
• C. 40
• D. 15
• E. 25

Answer 1

The correct option is B 20

Let A takes x hours, then B = (x + 40) hours
$\frac{1}{x}+\frac{1}{x+40}=\frac{1}{15}$
Solving, we get x = 20