Multiple choice general knowledge math & puzzles

Two pipes A and B can fill a tank in 36 hours and 45 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?

  1. 20 hrs

  2. 40 hrs

  3. 10 hrs

  4. 30 hrs

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

Both pipes fill the tank: A at 1/36 per hour and B at 1/45 per hour. Combined rate = 1/36 + 1/45 = (5+4)/180 = 9/180 = 1/20 per hour, so the tank fills in 20 hours. The other options do not match the combined rate.