Multiple choice

A tap can fill a cistern in 40 min and a second tap can empty the filled cistern in 60 min. By mistake without closing the second tap, the first tap was opened. In how many minutes will the empty tank be filled?

  1. 72

  2. 84

  3. 108

  4. 120
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Fill tap rate = 1/40 per min, empty tap rate = 1/60 per min. Net fill rate with both open = 1/40 - 1/60 = (3-2)/120 = 1/120 per min. Time to fill = 1 / (1/120) = 120 min. The empty tap drains slower than the fill tap fills, so tank still fills, just at reduced rate.

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