Question 710847
Add their rates of filling
Rate of 1st pipe = ( 1 tank filled ) / ( 6 hrs )
Rate of 2nd pipe = ( 1 tank filled ) / ( 10 hrs )
{{{ 1/6 + 1/10 = 1/t }}}
Multiply both sides by {{{ 30t }}}
{{{ 5t + 3t = 30 }}}
{{{ 8t = 30 }}}
{{{ t = 3.75 }}}
If both pipes are open the tank is filled in 
3 hrs 45 min