Answer:there are 21 fish in the tank.
Explanation:
Let's assume that the total number of fish in the tank is x.
We know that 6/7 of the fish have a red stripe on them. Therefore:
(6/7)x = number of fish with red stripes
We also know that the number of fish with red stripes is 18. Therefore:
(6/7)x = 18
Multiplying both sides of the equation by (7/6), we get:
x = 21