The population of their school of fish was reduced by a percentage of 20%.
Percentage is defined as part of a whole expressed in hundredths or out of 100. It is usually written with the symbol "%".
First, determine how much was reduced in the number of fish by subtracting 360 fishes from 450 fishes.
decrease in number of fishes = 450 - 360
decrease in number of fishes = 90
Then, determine the percent that is reduced by dividing the decrease in number of fishes by the initial number of fishes and multiply by 100.
percent decrease = (90 / 450) x 100
percent decrease = 0.2 x 100
percent decrease = 20%
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