Question

I can only form 3 equations with 4 unknowns for this question

Answer 1

Me too. Does the problem ever tell you the total number of fish he wants to buy?

Answer 2

No

Answer 3

Apparently the answer key says:
5 guppies, 3 mollies, 2 goldfish and 1 neon tetra...

Answer 4

I tried reducing the system and got\[\begin{bmatrix}
1 & 0 & 0 & 0 & 5\\
0 & 1 & 0 & -1 & 2\\
0 & 0 & 25 & 22 & 72
\end{bmatrix}\]This may shed some light.

Answer 5

(Hint: solve accordingly)