You can solve this problem using ratios.
6/7 of the fish have a red stripe.
18 fish have a red stripe.
6:7 = 18:x
6/7 is just a simplified fraction of 18/x, both of which are equal.
In these fractions, the numerator represents the fish with red stripes and the denominator represents the total number of fish.
6:7 = 18:x
x = Total fish
The question is asking us to find x.
Since the fractions are equivalent and there is a known numerator in both of them, divide the numerator of 18:x, or 18/x, which is 18, by the numerator of 6:7, or 6/7, which is 6.
18 / 6 = 3
Now multiply 3 by the known denominator, which is 7.
7 • 3 = 21
6/7 = 6:7
18/x = 18:x
6:7 = 18:x
6:7 = 18:21
x = 21
There are 18 fish with red stripes.
There are 21 total fish.
6/7, or about 86%, of the fish have red stripes.
The answer is there is a total of 21 fish in the tank.
Hope this helps!