For this case we propose a system of equations:
x: Let the variable representing the quantity of Granny Smith apples
y: Let the variable representing the number of Gala apples
According to the statement we have:
From the first equation we have to:
replacing in the second equation:
So, Carl bought 7 Gala apples
On the other hand:
So, Carl bought 12 Granny Smith apples
Answer:
Carl bought 7 Gala apples and 12 Granny Smith apples