Question

One hose can fill a goldfish pond in 3030 ​minutes, and two hoses can fill the same pond in 2121 minutes. find how long it takes the second hose alone to fill the pond.

Answer 1

One hose can fill the pond in 30 minutes, which means every minute it fills 1/30 of the pond.
the second hose can fill the pond in x minutes, which means every minute it fills 1/x portion of the pond.
together it fills the WHOLE pond in 21 minutes: 21/30 +21/x=1
21/x=1- 21/30=9/30=3/10
x=70