`a) (13 3/7 + 4 5/13) - 8 3/7`
` = 13 3/7 + 4 5/13 - 8 3/7`
` = (13 3/7 - 8 3/7) + 4 5/13`
` = 5 + 4 5/13`
` = 9 5/13`
`b) 10/17 - 5/13 + 7/17 + -8/13 - 11/25`
` = (10/17 + 7/17) - (5/13 + 8/13) - 11/25`
` = 1 - 1 - 11/25`
` = 0 - 11/25`
` = -11/25`
`c) -7/25 . 11/13 + -7/25 . 2/13 - 18/25`
` = -7/25 . (11/13 + 2/13) - 18/25`
` = -7/25 . 1 - 18/25`
` = -7/25 - 18/25`
` = -1`
