`a) 1 + 1/3 + 1/6 + 1/9`
`=18/18 + 6/18 + 3/18 + 2/18`
`= \frac{18+6+3+2}{18}`
`=29/18`
`b) 3 - 1/2 - 1/4 - 1/6`
`=36/12 - 6/12 - 3/12 - 2/12`
`= \frac{36 - 6 - 3 - 2}{12}`
`= 25/12`
`c) 5/28 xx 77/10 : 22/15`
`=\frac{5 xx 77}{28 xx 10} xx15/22`
`=\frac{1 xx 11}{ 4xx2} xx 15/22`
`= 11/8 xx 15/22`
`= \frac{11xx15}{8xx22}`
`= \frac{1 xx 15}{8 xx2}`
`= 15/16`