Hi I’m having trouble with a math question. If someone could help me out on how to do the whole problem it would be greatly appreciated. The math problem is below:

Question

One pump can drain a pool in 5 minutes. When a second pump is also used, the pool only takes 4 minutes to drain the pool. How long would it take the second pump to drain the pool if it were the only pump in use?

Answer 1

T1*T2/(T1+T2) = 4.
5*T2/(5+T2) = 4,
5T2 = 20+4T2,
T2 = __Min. = Time for 2nd pump to drain pool.

Answer 2

in 4 minutes, the 1st pump drains 80% of the pool ... 4 / 5 = 80%

this means that in 4 minutes, the 2nd pump drained 1/5 of the pool
... draining the whole thing would take 5 times as long ... 4 * 5 = 20