AHSME 1991 Problem 16

One hundred students at Century High School participated in the AHSME last year, and their mean score was 100. The number of non-seniors taking the AHSME was $50\%$ more than the number of seniors, and the mean score of the seniors was $50\%$ higher than that of the non-seniors. What was the mean score of the seniors? (A) $100$ (B) $112.5$ (C) $120$ (D) $125$ (E) $150$