9/10 + 4/9 : (6 - 5 11/21) - 1 2/3 = 1/6
1) 6 - 5 11/21 = 5 21/21 - 5 11/21 = 10/21
2) 4/9 : 10/21 = 4/9 · 21/10 = (2·7)/(3·5) = 14/15
3) 9/10 + 14/15 = 27/30 + 28/30 = 55/30 = 11/6 = 1 5/6
4) 1 5/6 - 1 2/3 = 1 5/6 - 1 4/6 = 1/6
