Simple:
Of the 8 fish we get 30 are tagged, hence we can assume the ratio of tagged : non tagged is:
\frac{8}{30}
Let x equal the number of fish in the damn after the fish were added:
Ratio of the tagged: non tagged should be the same ->
\frac{8}{30}=\frac{50}{x} \\ \\ \therefore x=187.5...