`a) (-4)/5 xx 7/12 + (-4)/5 xx (-1)/6`
` = (-4)/5 xx (7/12 - 1/6)`
` = (-4)/5 xx 5/12`
` = -1/3`
`b) |-15/2| + 3/18 - (-5/15)^2`
` = 15/2 + 3/18 - 1/9`
` =135/18 + 3/18 - 2/18`
` = 68/9`
`c) (2020/4040 - 1/7) - (-3/21) + (-5)/12`
` = 1/2 -1/7 + 1/7 - 5/12`
` = 1/2 - 5/12`
` = 6/12 - 5/12`
` = 1/12`