1) 8,5 - 1/3 = 85/10 - 1/3 = (3*85 - 10*1)/30 = (255-10)/30 = 245/10 = 24,5
2) 4 1/9 + 1,8 = 37/9 + 18/10 = (10*37 + 9*18)/90 = (370 + 162)/90 = 532/90 = 266/45 = 5 41/45
3) 1/9 * 0,12 = 1/9 * 12/100 = 1/75
4) 6/7 : 0, 6 = 6/7 : 6/10 = 6/7 * 10/6 = 10/7 = 1 3/7