Multiple choice

Pipe $A$ can fill an empty tank in $5$ hours while Pipe $B$ can empty the full tank in $6$ hours. If both are opened in the empty tank, time taken to fill it up completely is __________.

  1. $30\ hours$
  2. $20\ hours$
  3. $10\ hours$
  4. $40\ hours$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Pipe A fills at 1/5 per hour, Pipe B empties at 1/6 per hour. Net rate = 1/5 - 1/6 = 1/30. Time to fill = 30 hours.

AI explanation

Using the combined work formula, add the hourly rates of 1 divided by 5 for the filling pipe and minus 1 divided by 6 for the emptying pipe. This results in a net rate of 6 divided by 30 minus 5 divided by 30, which equals 1 divided by 30. Inverting this net rate gives a total filling time of 30 hours.

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