The answer i don't know how to do it either if you could me that would be

70% of the fish are swordtail fish.

30% of 90 is 27, so there are 27 rainbow fish.

70% of 90 is 63, so there are 63 swordtail fish.

It is very simple to figure this question out. It is a little difficult to explain for me, but I will try as best I can.

90 fish is the most amount of fish allowed in the tank, so that means it's 100% of fish in a tank. Since only 30% of the fish are rainbow, that means the leftover has to be swordtail. So you do the math:
100% - 30% = 70%

Now for this part, you have to figure out how many fish are equal to those percentages. 90 is an easy number in this equation.

Put a decimal at the end of 90
It becomes 90.
90. is the same as 90 so nothing has changed. Now you're going to move the decimal over to the left one time. The number 90 now becomes 9.0, or just 9

We take the 30% and get rid of the zero on the end so it becomes a 3. Now you multiply 9 times 3, which equals 27. So, you have 27 rainbow fish.

Do the same with the 70% as well. Get rid of the zero at the end so it's just 7 and multiply it by 9. 7x9=63

If you want to check your work, you can add 27 + 63, and that equals 90. Since 90 is the most amount of fish you can have in the tank, you know you have the right answer.

I apologize if this was confusing. It took me a while to understand this stuff, but once I got the hang of it, it was very simple. Good luck on your work!

D. i think it is the savings