15 cats, 10 have stripes, 7 have stripes and green eyes, how many cats have just green eyes

[math_celebrity]

Let G be green eyes and S be Stripes, and SG be Stripes and Green Eyes.

Set up an equation
Total Cats = Green Eyes + Stripes - Green Eyes and Stripes
15 = G + 10 - 7
15 = G + 3

Subtract 3 from each side:
G = 12