David estimated he had about 20 fish in his pond. A year later, there were about 1.5 times as many fish. The year after that, the number of fish increased by a factor of 1.5 again. The number of fish is modeled by f(x) = 20(1.5)^x.

Using this question what logarithim would I make to find out how many years it would take for David to have 60 fish in his pond?

Also could you walk me through how it works?