`a, 1/4 + ( -5/13 ) = 13/52 - 20/52`
`= -7/52`
`b, -16/7 + 21/31 = 21/31 - 16/7`
`= 147/217 - 208/217`
`= -61/217`
`c, -20/35 + 16/24 = -4/7 + 2/3`
`= 2/3 - 4/7`
`= 14/21 - 12/21`
`= 2/21`
`d, -21/77 + ( -10/35 ) = - 3/11 - 10/35`
`= -105/385 - 110/385`
`= -215/385 = -43/77`