Whole-number solutions

A shop packs pens in boxes of $5$ and boxes of $3$. How many ways can it fill an order of exactly $90$ pens? That is the number of whole-number solutions of $5x + 3y = 90$, with $x$ and $y$ both $0$ or more.

The idea. Try every $y$ from $0$ up to $\dfrac{c}{b}$. Keep $y$ when $c - by$ divides evenly by $a$.

Your tasks.

1. In fits, return True when a * x + b * y equals c.
2. In whole_solutions, return the list of pairs (x, y) with $x$ and $y$ whole and $0$ or more.
3. Set ways to the number of ways to pack the $90$ pens.