Pipe $A$ alone fills a tank in $6$ hours and pipe $B$ alone in $9$ hours. Both opened together fill it in:

Pipe $A$ alone fills a tank in $6$ hours and pipe $B$ alone in $9$ hours. Both opened together fill it in:
  1. $3$ hr
  2. $\dfrac{18}{5}$ hr
  3. $\dfrac{15}{4}$ hr
  4. $5$ hr

Solution

Combined rate $= \dfrac{1}{6} + \dfrac{1}{9} = \dfrac{3 + 2}{18} = \dfrac{5}{18}$/hr. Time $= \dfrac{18}{5}$ hr.

Asked in: IMO

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