Answer: 21
Step-by-step explanation: We know that 6/7 of the fish in the tank have red stripe on them, and the exact number is 18.
This means that, of a number of total fish, X, when we multiply it by 6/7, we obtain 18 as result:
6/7*X = 18
X = 18*7/6 = 21
So we have a total of 21 fish in the tank, where 18 red fish have a red stripe, and the other 3 are different.