Concept: The question tests on Rates
Let the total time taken to read = t
80M + 100D = t ... (1) and
50M + 250D = t ... (2)
Time taken to read 80 math pages and 100 drawing pages = time taken to read 50 math pages and 250 drawing pages.
80M + 100D = 50M + 250D
30M = 150D
Or D = 30/150=1/5 M
Substituting in equation (1), 80M + 100 * 1/5 M = 80M + 20M = 100M
So he can read 100 math pages in the given period of time.