`a) -1/2 - \frac{-3}{5} + \frac{-1}{4}`
`= -10/20 + 12/20 + \frac{-5}{20}`
`= \frac{-10+12+(-5)}{20}`
`= -3/20`
--------------------------------------------
`b) \frac{-5}{6} . 4/19 + \frac{-5}{6} . 15/19 + 20 5/6`
`= \frac{-5}{6} . ( 4/19 15/19) + 125/6`
`= \frac{-5}{6} . 1 + 125/6`
`= \frac{-5}{6} + 125/6`
`= 20`
-----------------------------------------------
`c) -4/17 . 3/5 . 17/4 . (-20) . 5/-36`
`= (-4/17 . 17/4) . ( 3/5 . 5/-36) . (-20)`
`= (-1) . (-1/12) . (-20)`
`= [(-1) . (-20)] . (-1/12)`
`= 20 . (-1/12)`
`= -5/3`
---------------------------------------------------
`d) 6/7 . \frac{-21}{30} + 5/8 : 5 - 3/16 . (-2)^2`
`=-3/5 + 1/8 - 3/4`
`=-24/40 + 5/40 - 30/40`
`= \frac{-24+5-30}{40}`
`= -49/40`
