Question 920101
A pool can be filled by pipe A in 3 hours and by pipe B in 5 hours.
 When the pool is full, it can be drained by pipe C in 4 hours.
 If the pool is initially empty and all three pipes are open,
 write an equation to find how long it takes to fill the pool.
:
Let t = time required to fill the pool with all valves open
Let the completed job = 1 (a full pool)
Fill is +, drain is -
:
{{{t/3}}} + {{{t/5}}} - {{{t/4}}} = 1
multiply equation by 60 to cancel the denominators, find t