g + b = 8173, (1) g - b = 421. (2) Every time when you see two equations like these, add them. In your case you get 2g = 8173 + 421, or 2g = 8594. Hence, g == 4297. There are 4297 girls in the college. Now, from (1), b = 8173 - g = 8173 - 4297 = 3876. Answer. 4297 girls and 3876 boys.