`a) -15/14 : | 21/-10|`
`= -15/14 : 21/10`
`= -15/14 . 10/21`
`= -25/49`
------------------------------------------------
`b) 1/7 - ( 3/5 - 4/42) - | 2/3 - 1/2|`
`=1/7- 53/105 - 1/6`
`= -38/105 - 1/6`
`= -37/70`
--------------------------------------------------
`c) 4,2 - | -4/5| + 1/2`
`= 21/5 - 4/5 + 1/2`
`= 17/5 + 1/2`
`= 39/10`
----------------------------------------------
`d) 0,75 - 2/3 - (-0,5)`
`= 3/4 - 2/3 + 1/2`
`= 9/12 - 8/12 + 6/12`
`= \frac{9-8+6}{12}`
`= 7/12`
