8/17 + 4/17 - 9/17 = (8 + 4 - 9)/17 = 3/17

4 7/23 - 2 5/23 + 7 15/23 = (4 - 2 + 7) + (7/23 - 5/23 + 15/23) = 9 + 17/23 = 9 17/23