One hose can fill a goldfish pond in 30 ​minutes, and two hoses can fill the same pond in 21 minutes. Find how long it takes the second hose alone to fill the pond.

Answer 1

Assume that the pond holds 3000 gallon (this is arbitrary... any reasonable number is good)

If the first hose fills in 30 minutes, the rate is 3000 gal/30 min = 100 gal/min

Then to fill the pond in 21 minutes, the total flow rate is 3000 gal/21 min = 142.857 gpm

This means the 2nd hose flow rate is 142.857 - 100 = 42.857 gpm

So, to fill with the 2nd hose: 3000 gal/42.857 gal/min = 70 minutes