a) `-3/17 . 28/55`
`= -84/935`
b) `-5/9 : (-7/18)`
`=-5/9 . (-18/7)`
`=90/63`
`=10/7`
c) `5/6 .(-12/7) : 15/21`
`= -60/42 : 5/7`
`= -10/7 . 7/5`
`= -2`
d) `5/9 : ( 1/11 - 5/22) + 5/9 : ( 1/15 - 2/3)`
`= 5/9 : ( 2/22 - 5/22) + 5/9 : ( 1/15 - 10/15)`
`= 5/9 : (-3/22) + 5/9 : (-3/5)`
`= 5/9 . ( -22/3) + 5/9 .(- 5/3)`
`= 5/9 [ -22/3 + (-5/3)]`
`= 5/9 . (-9)`
`=-5`
