Two pipes $A$ and $B$ together fill a tank in $12$ minutes. If $A$ alone takes $20$ min, then $B$ alone takes:

Two pipes $A$ and $B$ together fill a tank in $12$ minutes. If $A$ alone takes $20$ min, then $B$ alone takes:
  1. $24$ min
  2. $28$ min
  3. $30$ min
  4. $32$ min

Solution

$B$'s rate $= \dfrac{1}{12} - \dfrac{1}{20} = \dfrac{5 - 3}{60} = \dfrac{2}{60} = \dfrac{1}{30}$. Time $= 30$ min.

Asked in: IMO

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