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.

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