Question 841581
Pipe A's rate of filling is {{{ 1/45 }}} pools/minute
Pipe C's rate of emptying is {{{ -1/30 }}} pools/minute ( minus for emptying )
Let B's rate of filling = {{{ 1/t }}}  pools/minute
-----------------------------------------
I can add the rates of the 3 pipes
{{{ 1/45 + 1/t - 1/30 = 1/30 }}}
Multiply both sides by {{{ 90t }}}
{{{  2t + 90 - 3t = 3t }}}
{{{ 4t = 90 }}}
{{{ t = 22.5 }}}
Pipe B alone can fill the pool is 22.5 minutes
check answer:
{{{ 1/45 + 1/22.5 - 1/30 = 1/30 }}}
{{{ .0222 + .0444 - .0333 = .0333 }}}
{{{ .0666 - .0333 = .0333 }}}
{{{ .0666 = .0666 }}}
OK