10/5 = 1/x
cross multiply
10x = 5
x = 5/10
x = 1/2...u need 1/2 gallon per fish
The correct answer is the first option.
If you want to use elimination, you can sum the two equations for example, so that the x's simplify:



Plug this value for y in one of the equations to derive the value of x:

So, the solution is 