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?

He brought 7 zebra fish

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