Two pipes A and B together can fill a tank in 40 minutes. Pipe A is twice as fast as pipe B. Pipe A alone can fill the tank in:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 1 hour
Let pipe B fill r of the tank per minute (the part of the tank filled per minute); then A fills 2r, and together they fill 3r:
\( \displaystyle 3r = \frac{1}{40} \)
\( \displaystyle 2r = \frac{2}{3} \times \frac{1}{40} = \frac{1}{60} \)
Pipe A alone fills 1/60 of the tank per minute, so it takes 60 minutes = 1 hour.