`a) (-4)/7 - 5/13 . (-39)/25 + (-1)/42 : (-5/6)`
`= (-4)/7 - (-3/5) + 1/35`
`= -20/35 + 21/35 + 1/35`
`= 2/35`
``
`b) 2/9 . [ (-4)/45 : ( 1/5 - 2/15) + 1 2/3] - (-5)/27`
`= 2/9 . [ (-4)/45 : 1/15 + 5/3] + 5/27`
`= 2/9 . [-4/3 + 5/3] + 5/27`
`= 2/9 . 1/3 + 5/27`
`= 2/27 + 5/27`
`= 7/27`