a) `8/15 .x + 5/7 = -1/21`
`8/15 .x = -1/21 - 5/7`
`8/15 .x = -16/21`
`x = -16/21 : 8/15`
`x = -10/7`
b) `(5x+3/2 ) : 8/15 + 5/6 = 2 1/12`
`(5x+3/2) : 8/15 = 25/12 - 5/6`
`(5x+3/2) : 8/15 = 5/4`
`5x + 3/2 = 5/4 . 8/15`
`(5x+3/2) = 2/3`
`5x = 2/3 - 3/2`
`5x = -5/6`
`x = -5/6 : 5
`x = -1/6`
c) `x - 40% .x = -0,6`
`(1 - 2/5) .x = -0,6`
`3/5 x = -0,6`
`x= -0,6 : 3/5`
`x=-1`