12 3/5:[5 2/15+(5 5/9-8 3/4:(x:8/21-1 2/5)*5 68/125]-2/11=4/5
12 3/5:[5 2/15+(5 5/9-8 3/4:(x:8/21-1 2/5)*5 68/125]=4/5+2/11=44/55+10/55=54/55
12 3/5:[5 2/15+(5 5/9-8 3/4:(x:8/21-1 2/5)*5 68/125]=54/55
[5 2/15+(5 5/9-8 3/4:(x:8/21-1 2/5)*5 68/125]=12 3/5:54/55=63/5:54/55=63*55/5*54=7*11/1*6=77/6=12 5/6
5 2/15+(5 5/9-(8 3/4:(x:8/21-1 2/5))*5 68/125=12 5/6
(5 5/9-(8 3/4:(x:8/21-1 2/5))*5 68/125=12 5/6-5 2/15=77/6-77/15=385/30-154/30=231/30=77/10
(5 5/9-(8 3/4:(x:8/21-1 2/5))*5 68/125=77/10
(5 5/9-(8 3/4:(x:8/21-1 2/5))=77/10:5 68/125=77/10:693/125=77*125/10*693=1*25/2*9=25/18
5 5/9-(8 3/4:(x:8/21-1 2/5)=25/18
8 3/4/(x:8/21-1 2/5)=5 5/9-25/18=50/9-25/18=100/18-25/18=75/18=25/6
8 3/4/(x:8/21-1 2/5)=25/6
x:8/21-1 2/5=8 3/4:25/6=35/4:25/6=35*6/4*25=7*3/2*5=21/10
x:8/21-1 2/5=21/10
x:8/21=21/10+1 2/5=21/10+7/5=21/10+14/10=35/10=7/2
x:8/21=7/2
x=7/2*8/21=7*8/2*21=1*4/1*3=4/3=1 1/3
Ответ: х=1 целая 1/3