Greta can read 0.2 (a fifth) of a page per minute
So, we know it takes Greta 5 hours to read 60 pages a day because 900/15 is 60. 5 hours is 300 minutes, and 60/300 is 0.2. Therefore, Greta can read 0.2 or a fifth of a page per minute, or one page per five minutes.