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