a, `7/12 . 3 2/5 -7/12 . 9 2/5`

`=7/12 . 17/5 - 7/12 . 47/5`

`=7/12.(17/5-47/5)`

`=7/12 . (-6)`

`=-7/2`

b, `2 6/31-5/24+25/31-1 1/24+0,25`

`=2+6/31-5/24+25/31-1-1/24+1/4`

`=(2-1)+(6+31+25/31)+(-5/24-1/24+1/4)`

`=1+1+(-6/24+6/24)`

`=2`

c, `1 8/15:(-3/5)-5 11/15:(-3/5)`

`=23/15 . (-5)/3 - 86/15 . (-5)/3`

`=(-5)/3 . (23/15-86/15)`

`=(-5)/3 . ()`

`=(-5)/3 . (-63)/15`

`=5/3 . 21/5`

`=7`