20- (13 17/35- Х)= 13 27/28
19 35/35- 13 17/35+ Х= 13 27/28
6 18/35+ Х= 13 27/28
Х= 13 27/28- 6 18/35
Х= 13 (27•5)/(28•5)- 6 (18•4)/(35•4)
Х= 13 135/140- 6 72/140
Х= 7 63/140= 7 9/20
Проверка
20- (13 17/35- 7 9/20)= 20- 13 17/35+ 7 9/20= 27 9/20 - 13 17/35= 27 (9•7)/(20•7) - 13 (17•4)/(35•4)= 27 63/140 - 13 68/140= 26 (140•1+63)/140- 13 68/140= 26 203/140- 13 68/140= 13 135/140= 13 27/28