105 - [(12 целых 1/2+28 целых 6/7) - (19/21 + 34 целых 5/21)] - (103 целых 4/21 - 72 целых 5/18)

105 - [(12 1/2+28 6/7) - (19/21 + 34 5/21)] - (103 4/21 - 72 5/18)=
   =105 - [(12 7/14+28 12/14) - 34 24/21] - (103 4/21 - 72 5/18)=
   =105 - [41 5/14 -35 3/21] - (103 24/126 - 72 35/126)=
   =105 - [41 5/14 -35 1/7] - (31 24/126 -24/126-11/126)=
   =105 - [41 5/14 -35 2/14] - (31 -11/126)=105 - 6 3/14 - (30 115/126)=
   =105 - 6 27/126 - 30 115/126=105-36 142/126=105-37 16/126=
   =105- 37 8/63=67  55/63