Question 915624
Add the rates of filling and subtract
the rate of draining
{{{ 1/30 + 1/20 - 1/15 = 1/t }}}
Multiply both sides by {{{ 60t }}}
{{{ 2t + 3t - 4t = 60 }}}
{{{ t = 60 }}}
It will take 1 hour with both filling pipes 
open and the drain open