One pump can drain a pool in 10 minutes. When a second pump is also used, the
pool only takes 4 minutes to drain. How long would it take the second pump to
drain the pool if it were the only pump in use?
Let the answer be "The second pump takes x minutes to drain the pool if it
were the only pump used."
One pump can drain a pool in 10 minutes.
Therefore the first pump's draining rate is 1 pool per 10 minutes.
That's 
The other pump can drain 1 pool in x minutes.
Therefore the second pump's draining rate is 1 pool per x minutes.
That's 
When a second pump is also used, the pool only takes 4 minutes to drain.
So their combined draining rate is 1 pool per 4 minutes.
That's 
The equation comes from
Multiply through by LCD = 20x and solve for x.
[I won't tell you the answer but it's a fraction and it's more than 4
but less than 10.]
Edwin
