there are 15 guppies, 14 angelfish and 5 goldfish in a tank. how many guppies and goldfish must be added so that the ratio of guppies to angelfish to goldfish is 9:7:5

From the way the question is stated, you want the number of angelfish to remqin the same (14). You want 5/7 as many goldfish, which would be ten. So you add five goldfish to the five you started with.

Similarly, you want the number of guppies to be 9/7 of 14, which is 18. So you add three guppies.
You then have a ratio 18:14:10, which equals 9:7:5