答案：D

解析：

Among the eighty seniors, there are 3 boys for every 5 girls, so 3/8 of the seniors, or 30, are boys and 5/8, or 50, are girls. Among the juniors, 3/5 are boys and 2/5 are girls. If x stands for the total number of juniors, then (3/5)x are boys and (2/5)x are girls. The total number of senior and junior boys is 30+((3/5)x). The total number of senior and junior girls is 50+((2/5)x). The question states that these quantities are equal, so 30+((3/5)x) = 50+((2/5)x). Solving this gives 150 + 3x = 250 + 2x, or x = 100.