`a) (-0,25). 4/17 . (-3 5/21) . (-7/23)`
`= `$\dfrac{-1}{4}$`.4/17 . (-68/21) . (-7/23)`
`= `$\dfrac{-1}{17}$` . 68/69`
`= -4/69.`
`b) (-2/5) . 4/15 + (-3/10) . 4/15`
`= 4/15 . [(-2/5) + (-3/10)]`
`= 4/15 . [(-4/10) + (-3/10)]`
`= 4/15 . (-7/10)`
`= -14/75.`
`c) 21 - 3 3/4 : (3/8 - 1/6)`
`= 21 - 15/4 : (18/48 - 8/48)`
`= 21 - 15/4 : 5/24`
`= 21 - 15/4 . 24/5`
`= 21 - 18`
`= 3.`
`d) (`$\dfrac{-5}{6}$` + 2/5) : 3/8 + (4/5 - 11/30) : 3/8`
`= (`$\dfrac{-25}{30}$` + 12/30) × 8/3 + (24/30 - 11/30) × 8/3`
`= (-13/30) × 8/3 + 13/30 × 8/3`
`= 8/3 × (13/30 - 13/30)`
`= 8/3 × 0`
`= 0.`