Answer:
Let B = the weight of box B
Let 2B = the weight of box A
Let 2B + 30 = the weight of box C
B + 2B + 2B + 30 = 640
5B = 610
B = 122 pounds, weight of box B
2B = 2(122) = 244 pounds, weight of box A
2B + 30 = 2(122) + 30 = 274 pounds, weight of box C
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