21-6/7-(3 15/28 / 3 9/56 - 1 / 1 10/49)/2=
= 20 1/7 -(3 30/56 : 3 9/56 -1/ 1 10/49)/2=
=20 1/7 -(204/56 / 177/56 - 1 / 59/49)/2=
= 20 1/7 - (204/56/ 56/177 - 1 / 49/59)/2=
=20 1/7-(1 9/59 - 49/59)/2=
=20 1/7-0.2 / 2=
= 20 1/7 -0.1=20 1/7- 1/10=
=20 10/70-7/10=20 3/70