Cardinal Number Problem

I’m studying a bit, and I’ve come across this problem that I’m stuck on. It’s the last one in a set of 50 and it’s driving me nuts!

Find the indicated cardinal number.
28) Find n(G), given that n(D × G) = 20 and D = {7, 8, 9, 10}.
A) 9 B) 5 C) 4 D) 24

I’m leaning toward A for some reason because if you cont 10 more from 10 in the set, the cardinal number would be 10, but I can’t seem to find out if the formula consists of a -1.
If anyone could please explain this to me, it would be greatly appreciated. Thanks.

The symbol x for Cartesian Product should give you a clue. How many elements in {a,b} x {1,2,3}? How many in {x,y,z} x {f,o,u,r}? Just list all the elements and you’ll see the pattern. Then your problem will be easy.