Final answer:
To make 30 grams of box C, we would need 3 boxes of A.
Step-by-step explanation:
To make box C, one A and one B are required. According to the information given, the net mass of one A is 10 grams and the net mass of one B is 5 grams. So to make 30 grams of box C, we would need 3 boxes of A (3 x 10 grams = 30 grams).