Final answer:
The ratio of pink to blue fish in the tank, given that there are 21 fish in total and 9 of them are pink, is 3:4.
Step-by-step explanation:
To find the ratio of pink to blue fish, we need to determine the number of pink and blue fish in the tank. Given that there are 21 fish in total, and 9 of them are pink, we can subtract 9 from 21 to find the number of blue fish: 21 - 9 = 12. So, there are 12 blue fish in the tank. To simplify the ratio, we divide both the number of pink and blue fish by their greatest common factor, which is 3. The simplified ratio of pink to blue fish is therefore 3:4.