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

20-18,6:(6 11/15-4 3/20)=
1) 6 11/15-4 3/20= 101/15-83/20=404/60-249/60=155/60
2) 18,6:155/60=186/100*60/155= 11160/15500=72/100=0,72
3) 20-0,72=19,28

1.101/15-83/20=404-249/60=155/60=2,58. 2.18,6:2,58=7,2 3.20-7,2=12,8