So. 2/5 of the fish are surgeon fish. Meaning that 40% of the fish are surgeonfish. From that 40%, the 75% are yellow tangs. So the 75% of that 40% equals to 30% of all fish (since (3/4)*(2/5)=3/10)
So the aquarium has 30% OR 3/10 of its fish being yellow tangs