Question 674942
How much of the pool is filled by
pipe A in 1/2 hr ?
( 1 pool /5 hrs ) x ( 1/2 ) = 1/10 of a pool
There is now 9/10 of the pool left to fill
Let {{{ t }}} = the time to fill 9/10 of the pool
Used together, the rates of A and B add
{{{ 1/5 + 1/4 = (9/10) / t }}} 
Multiply both sides by {{{ 20t }}}
{{{ 4t + 5t = 18 }}}
{{{ 9t = 18 }}}
{{{ t = 2 }}}
The total time to fill the pool is 2 and 1/2 hrs
When A and B are filling together, they take 2 hrs to finish