`a, 12/5 + 3/6`
`= 72/30 + 15/30`
`=87/30`
`b, 3/9 + 25,6 - 7,4`
`= 3/9 + 128/5 - 37/5`
`= 15/45 + 1152/45 - 333/45`
`= 1167/45 - 333/45`
`= 834/45`
`= 278/15`
`c, 3/9 . 7,4 + 7/4 . 5/9 - 7/4 . 5/9`
`= 3/9 . 37/5 + 5/9 . (7/4 - 7/4)`
`= 111/45 + 5/9 . 0`
`= 37/15 + 0`
`= 37/15`