答案：D

解析：

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