Question 670005
Add their rates of filling to get rate 
with both pipes open
1st pipe's rate:
( 1 tank ) / ( 10 hrs )
2nd pipe's rate: 
( 1 tank ) / ( x hrs )
Both together:
( 1 tank ) / ( 6 hrs )
----------------
{{{ 1/10 + 1/x = 1/6 }}}
Multiply both sides by {{{ 30x }}}
{{{ 3x + 30 = 5x }}}
{{{ 2x = 30 }}}
{{{ x = 15 }}}
The 2nd pipe alone can fill the tank in 15 hrs