`a, (-2/3 + 3/7) : 4/5 + (-1/3 +4/7) : 4/5`
`= -2/3 . 5/4 + 3/7 . 5/4 + -1/3 . 5/4 + 4/7 . 5/4`
`= 5/4 . ( -2/3 + -1/3 + 3/7 + 4/7 )`
`= 5/4 . ( -1 + 1)`
`= 5/4 . 0`
`= 0`
`b, 5/9 : ( 1/11 - 5/22) + 5/9 : ( 1/15 - 2/3 )`
`= 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`
`= -45/9`
`= -5`