There are no tanks with less than 30 fish, so the first two groups ( 20-24) and (25-29) aren't used.
There are 3 tanks with 30-34 fish.
There are 3 tanks with 35-39 fish.
There are 3 tanks with 40-44 fish.
There are 1 tanks with 45-49 fish.
Now drag the bars for each group to the number of tanks.
See attached picture:
