Question 751260
Let {{{ t }}} = time in hrs for both pipes
to fill {{{ 2/3 }}} of the tank
Add their rates of filling to get rate filling together
( 1 tank filled / 3 hrs ) + ( 1 tank filled / 2 hrs ) = ( 2/3 tank / t hrs )
{{{ 1/3 + 1/2 = ((2/3) / t) }}}
Multiply both sides by {{{ 6t }}}
{{{ 2t + 3t = 6*(2/3) }}}
{{{ 5t = 12/3 }}}
{{{ 5t = 4 }}}
{{{ t = 4/5 }}}
{{{ (4/5)*60 = 48 }}}
It will take 48 minutes for 
both pipes to fill 2/3 of the tank 
check:
{{{ 1/3 + 1/2 = ((2/3) / t) }}}
{{{ 1/3 + 1/2 = (2/3) / (4/5) }}}
{{{ 1/3 + 1/2 = (2/3)*(5/4) }}}
{{{ 1/3 + 1/2 = 5/6 }}}
Multiply both sides by {{{ 6 }}}
{{{ 2 + 3 = 5 }}}
OK