Let number of pages typed by C per hour = x
Hence number of pages typed by A per period of 4 hours = x
Hence number of pages typed by C per day = 8x [ as each of A , B and C work 8 hours per day ]
Number of pages typed by A per day = 2x [ as A types x pages per 4 hours ]
Now let pages typed by B per day be equal to 'y' ..
According to the question :
Pages by C - Pages by B = Pages by B - Pages by A
==> 8x - y = y - 2x
==> 2y = 10x
==> y = 5x
Thus total number of pages printed by A , B and C together in a day = 2x + 5x + 8x = 15x
Hence in 20 days , number of pages they will print = 300x
And give also number of pages they print in 20 days = 900
Hence
300 x = 900
==> x = 3
Hence number of pages typed by C per hour = x = 3