Subtract the percent of salmon that passed through the dam from 100 %.
20 %
Practice makes perfect
We know that 80 % of salmon pass through a single dam unharmed. This percent is the number of salmon that passed through the dam unharmed divided by the total number of salmon.
80 % = salmon that passed through dam/all salmon
The total number of salmon corresponds to 100 % of all salmon. We know that 80 % of them passed through the dam. The rest of them, that is 100 % - 80 % = 20 % of the salmon, did not pass through the dam unharmed. This means that the number of salmon decreased by 20 % when passing through the single dam.
