2,8 = 28/10 = 14/5
3/7 : 9/14 = 3/7 * 14/9 = 2/3
14/5 - 2/3 = (42 - 10) / 15 = 32/15
5 - (32/15) * 1,5 = 5 - (32/15) * (3/2) = 5 - 16/5 = (25 - 16) / 5 = 9/5 = 1,8