`a, 17/18 - 11/6 + 2/9`
` = 17/18 - 33/18 + 4/18`
` = -12/18`
` = -2/3`
`b. 8/15 + -7/18 - 13/90`
` = 48/90 + (-35)/90 - 13/90`
` = 0/90 = 0`
`c. 5/12 + -11/15 - 49/60`
` = 25/60 + -44/60 - 49/60`
` = -68/60 = -17/15`
`d. 21/28 - 18/60 + 3/5`
` = 3/4 - 3/10 + 3/5`
` = 15/20 - 6/20 + 12/20`
` = 21/20`