Answer:
6 packs, 9 packs
Explanation:
The ratio of mechanical pencil to free eraser is 2:4, or 1:2.
So that means, for every free eraser you get, you bought the double amount of pencils.
12 free erasers x 2 = 24 pencils
18 free erasers x 2 = 36 pencils
And a pack of pencils contains 4.
24/4 = 6 packs
36/4 = 9 packs