20-18.6: (6 11/15 - 4 3/20)

6 11/15-4 3/20=6-4+11/15-3/20=2+44/60-9/60=2+35/60=2 35/60=2 7/1218,6:2 7/12=18 3/5:2 7/12=93/5:31/12=93/5*12/31=36/5=7,220-7,2=12,8