Question 647194
Add their rates of filling:
( 1 tank / 40 min ) + ( 1 tank / 60 min ) = 1 tank / t min 
{{{  1/40 + 1/60 = 1/t }}}
Multiply both sides by {{{ 120t }}}
{{{ 3t + 2t = 120 }}}
{{{ 5t = 120 }}}
{{{ t = 24 }}}
The tank will be filled in 24 min with both pipes running