9 less than the product of a number q and 5 is r

9 - 5q = r

Since there are two unknown variable namely q and r. We can't solve any of the unknown by providing a number. Instead we can provide the formula on how to look for its corresponding value.

We know that:

r = 9 - 5q

9 - 5q = r

-5q = r - 9

-5q / - 5 = (r - 9) / - 5

q = (r - 9) / - 5