Multiple choice

If a pipe can fill a cistern in 8 hours and another pipe can empty the cistern in 10 hours, then what part of the cistern will be filled in 1 hour, if both pipes are opened simultancously?

  1. $\frac { 1 }{ 40 } $
  2. $\frac { 1 }{ 30 } $
  3. $\frac { 1 }{ 35 } $
  4. $\frac { 1 }{ 8 } $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The filling rate is 1/8 of the cistern per hour, while the emptying rate is 1/10 per hour. Their net rate is 1/8 - 1/10 = 1/40 of the cistern per hour.

AI explanation

Using the net work formula, subtract the emptying rate from the filling rate to find the portion filled in one hour. The first pipe fills 1/8 of the cistern in one hour, while the second pipe empties 1/10 of the cistern in one hour. The net part filled is 1/8 minus 1/10, which equals 5/40 minus 4/40, resulting in 1/40.

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