$17$) `(-3/4 + 2/5) : 3/7 + (3/5 + {-1}/4) : 3/7`
`= 7/3 . (-3/4 + 2/5 + 3/5 - 1/4)`
`= 7/3 . (-1 + 1)`
`= 7/3 . 0`
`= 0`
$18$) `(-3/4 + 2/3) : 5/{11} + (-1/4 + 1/3) : 5/{11}`
`= 5/11 . ( -3/4 + 2/3 - 1/4 + 1/3)`
`= 5/11 . (-1 + 1)`
`= 0`
$19$) `5/9:(1/{11} - 5/{11} ) + 5/9: (1/{15} - 2/3)`
`= 5/9 : {-4}/{11} + 5/9 : {-3}/{5}`
`= 5/9 .{-53}/{12}`
`= -{265}/{108}`
$20$) `7/8 : (2/9 - 1/18) + 7/8 : (1/36 - 5/15)`
`= 7/8 : 1/6 + 7/8 : {-11}/{36}`
`= 7/8 . {30}/{11}`
`= {105}/{44}`
