`a) 10/19 - 5/21 + 9/19 - 16/21`
`= ( 10/19 + 9/19) - ( 5/21 + 16/21)`
`= 1 - 1`
`= 0`
``
`b) (-1)/4 + 11/9 - 1/8 + 7/9`
`= ((-1)/4 - 1/8) + ( 11/9 + 7/9)`
`= -3/8 + 18/9`
`= 13/8`
``
`c) 12/17 - 19/20 + 5/17 - 1/20`
`= ( 12/17 + 5/17) - ( 19/20 + 1/20)`
`= 1 - 1`
`= 0`
``
`d) -3/2 + 11/8 - 1/4 + 5/8`
`= -12/8 + 11/8 - 2/8 + 5/8`
`= (-12 + 11 - 2 + 5)/8`
`=1/4`
``
`e) -4/15 + 14/23 - 11/15 + 9/23`
`= (-4/15 - 11/15) + ( 14/23 + 9/23)`
`= (-1) + 1`
`= 0`
``
`f) -1/2 + 7/10 - 1/9 - 1/18 + 3/10`
`= (-1/2 - 1/9 - 1/18) + ( 7/10 + 3/10)`
`= (-9/18 - 2/18 - 1/18) + 1`
`=-2/3 + 1`
`= 1/3`
