Multiple choice

Two pipes A and B fill a tank in 4 hours and 5 hours respectively. Both the pipes were filling the tank when after 60 minutes pipe A had some problem and was closed. How much time did it took to fill the tank completely?

  1. 2 hours 50 minutes

  2. 4 hours

  3. 3 hours 45 minutes

  4. 3 hours 30 minutes

Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Pipe A fills at 1/4 tank per hour, B at 1/5 tank per hour. Together they fill at (1/4 + 1/5) = 9/20 per hour. In 1 hour, they fill 9/20 of tank. Remaining = 11/20. Pipe B alone fills at 1/5 per hour, so time needed = (11/20) / (1/5) = 11/4 = 2.75 hours. Total time = 1 + 2.75 = 3.75 hours = 3 hours 45 minutes.