Question 970260
Add the pumping rates of the 2 pumps 
to get their rate working together
Rate for older pump:
[ 1 pool drained ] / [ 15 hrs ]
Rate for newer pump:
[ 1 pool drained ] / [ t hrs ]
Rate working together:
[ 1 pool drained ] / [ 6 hrs ]
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{{{ 1/15 + 1/t = 1/6 }}}
Multiply both sides by {{{ 30t }}}
{{{ 2t + 30 = 5t }}}
{{{ 3t = 30 }}}
{{{ t = 10 }}} 
The newer pump can drain the pool in 
10 hrs working on it's own
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check:
{{{ 1/15 + 1/t = 1/6 }}}
{{{ 1/15 + 1/10 = 1/6 }}}
{{{ 2/30 + 3/30 = 5/30 }}}
{{{ 2 + 3 = 5 }}}
{{{ 5 = 5 }}}
OK