The equation that would best describe the situation would be the first option. To solve this question, simply break down the question into sections and then use variables X and y to represent each of the things asked for in the problem. We know the following:
X = no of tetra fish
Y = no of goldfish
If there is twice as much goldfish than tetra fish, then it would be 2y.
X = 2Y.
Then knowing what X and y represent, find the equation that gives the total cost of fish he bought, knowing that 1.50 is for a goldfish and 2.00 is for a tetra fish.