`a ) 1/2 + ( - 3 )/4 + ( - 5 )/6`
`= 6/12 + ( - 9 )/12 + ( - 10 )/12`
`= ( - 3 )/12 + ( - 10 )/12`
`= ( - 13 )/12`
`b ) 7/15 - ( - 8 )/5 + ( - 1 )/3`
`= 7/15 - ( - 24 )/15 + ( - 5 )/15`
`= 31/15 + ( - 5 )/15`
`= 26/15`
`c ) 8/9 + ( - 5 )/6 + 7/2`
`= 16/18 + ( - 15 )/18 + 63/18`
`= 1/18 + 63/18`
`= 32/9`