Multiple choice

Two pipes can fill a tank in 18 minutes and 15 minutes. The outlet pipe can empty the tank in 45 minutes. If all the pipes are opened when the tank is empty, then how long will it take to fill the tank?

  1. 90 minutes

  2. 30 minutes

  3. 12 minutes

  4. 10 minutes

  5. 26 minutes
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Pipe A fills 1/18, Pipe B fills 1/15, Pipe C empties 1/45. Net rate = 1/18 + 1/15 - 1/45 = (5+6-2)/90 = 9/90 = 1/10. Time = 10 minutes.

AI explanation

Using the LCM method, let the tank capacity be 90 units. The two inlet pipes fill at rates of 5 and 6 units per minute while the outlet pipe empties at 2 units per minute. The net rate with all pipes open is 5 + 6 minus 2, equalling 9 units per minute. Dividing the total capacity of 90 units by the net rate of 9 gives a filling time of 10 minutes.

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