`c) 5/7 . 19/23 - 12/23 . 5/7`
`= 5/7 . ( 19/23 - 12/23)`
`= 5/7 . 7/23`
`= 5/23`
``
`c) (-10)/11 . 8/9 + 7/18 . 10/11`
`= 10/11 . (-8)/9 + 7/18 . 10/11`
`=10/11 . ((-8)/9 + 7/18)`
`= 10/11 . (-1/2)`
`= -5/11`
``
`c) 12/25 . 23/7 - 12/7 . 13/25`
`= 12/7 . 23/25 - 12/7 . 13/25`
`= 12/7 . ( 23/25 - 13/25)`
`= 12/7 . 2/5`
`= 24/35`