$A$ and $B$ together can do a job in $10$ days; $A$ alone in $15$ days. $B$ alone will take:

$A$ and $B$ together can do a job in $10$ days; $A$ alone in $15$ days. $B$ alone will take:
  1. $20$ days
  2. $25$ days
  3. $30$ days
  4. $35$ days

Solution

$B$'s rate $= \dfrac{1}{10} - \dfrac{1}{15} = \dfrac{3 - 2}{30} = \dfrac{1}{30}$. So $B$ alone takes $30$ days.

Asked in: IMO

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