Gary has three less pets than Abe. If together they own 15 pets, how many pets does Gary own?

Let g = Gary's pets and a = Abe's pets.

We are given two equations:
(1) g = a - 3
(2) a + g = 15

Substitute (1) into (2)
a + (a - 3) = 15

Combine Like Terms:
2a - 3 = 15

Add 3 to each side:
2a = 18

Divide each side by 2 to isolate a:
a = 9 --> Abe has 9 pets

Substitute a = 9 into Equation (1)
g = 9 - 3
g = 6 --> Gary has 6 pets