Assign variables randomly. So here, let's say the $2 fish are x and the $1 fish are y.
So we know that she spent $2 for every x and $1 for every y, and a total of $15.
2x + y = 15
Now, we also know that she bought 15 fish (is this right? because it's going to end up with a weird answer).
So x + y = 15.
Pick an equation and isolate a variable. I'll pick the second one and isolate x.
x = 15 - y
Now that you have that, you can plug x into the first equation.
This will give you y.
2(15-y) + y = 15
30 - 2y + y = 15
30 - y = 15
- y = -15
y = 15
Now that you have that, you can plug y into any equation to find the value of x.
x + (15) = 15
x = 0
So, she bought 15 of the $1 fish and none of the $2 fishies.
