Video Explanation

Text Explanation

Let

G = number of guppies

S = number of swordtails

Then, we are told

Thus, G:S = 8:9.  Guppies account for 8 “parts” of the fish, and swordtails account for 9 “parts.”  This means that, altogether, there must be 8 + 9 = 17 “parts” in the whole aquarium.  Thus, in the whole aquarium, guppies are 8 parts out of 17 parts, or of the whole aquarium.  

Answer = (B)