At the local pet store, zebra fish cost $2.00 each and neon tetras cost $2.15 each. if ernesto bought 15 fish for a total cost of $31.20, not including tax, how many zebra fish did he buy?

So first create and define your variables:
Z = amount of zebra fish
N = amount of neon tetras

Now create your equations:
2z+2.15n=31.20
z+n=15
This is your system. There are multiple methods to use but in this problem I’ll use the substitution method by simplifying the bottom equation.
2z+2.15n=31.20
z=15-n
Now I’ll plug the bottom equation into the top one.
2(15-n)+2.15n=31.20
And just solve from here.
30-2n+2.15n=31.20
0.15n=1.20
n=8
So he bout 8 neon tetras, and 15-8= 7, so he bought 7 zebra fish