... whose product satisfies what's needed. Like this:
18-64-c*d=4
18-64-4=c*d
c*d=-50
Then the student could pick any two numbers with product of -50, like c=-25 and d=2.
Another approach would be to pick any number for c, then express d in terms of that number. For example, if the student arbitrarily chose c=13 then:
18-64-c*d=4
18-64-13*d=4
d=(4-18+64)/(-13)