A basket with 12 apples has a mass of 3,105 grams. The same basket with 7 apples has a mass of 1,980 grams. Each apple has the same mass. What is the mass of the basket?

12 apples - 7 apples = 5 apples.
3,105 grams - 1,980 grams = 1125 grams.
So 5 apples weight 1125 grams.
1125 grams / 5 apples = 225 grams per apple.
12 apples x 225 grams each + basket = 3105 grams
2700 grams + basket = 3105 grams
basket = 3105 - 2700
basket = 405 grams