Add weight of first alloy and weight of second alloy. A + B = 300. So, the sum of weight is 300 grams. Copper = 0.70 A where 70 is 70 percent copper, and a is the weight of the first alloy. 0.40 b is the percentage of weight of the second alloy. 0.70 A + 0.40 B = 180 grams.