`a)(1/4 + -5/13) + ( 2/11 + -8/13 + 3/4 )`
`=1/4+(-5/13)+2/11+(-8/13)+3/4`
`=(1/4+3/4)+(-5/13+ -8/13)+2/11`
`=1+ -1+2/11`
`=2/11`
`b) (21/31 + -16/7 ) + (44/53 + 10/31 ) +9/53`
`=21/31+ -16/7 +44/53+10/31+9/53`
`=(21/31+10/31)+(44/53+9/53)+ -16/7`
`=1+1+ -16/7`
`=-2/7`
`c) 31/23 - ( 7/23 + 8/23 )`
`=31/23-15/23`
`=16/23`
`d) (1/3 + 12/67 + 13/41) - ( 79/67 - 28/41)`
`=1/3 + 12/67 + 13/41-79/67-28/41`
`=1/3+(12/67-79/67)+(13/41+28/41)`
`=1/3+ -1+1`
`=1/3`