Xiaojun reads a book. If he reads 80 pages a day, he needs 4 more days to finish it; if he reads 90 pages a day, he needs 3 more days to finish it; Page.
小军读一本书，如果每天读80页，需要4天多读完；如果每天读90页，需要3天多读完；如果每天读a页，刚好a天读完，则每天应读多少页．
<result>18</result>