Solo has to read 4 pages from his Science textbook, 20 pages from his Social Studies textbook, 7 pages from his History textbook and 8 pages from his Geography textbook. Solo read 15 pages on Monday. If he has 4  more days to complete all his reading, how many pages does he need to read, on average, in one day?
Please reason step by step, and put your final answer within \boxed{}.
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