4,2х+5,3у-2,6у-1,8х=(4,2х-1,8х)+(5,3у-2,6у)=2,4х+2,7у
354=300+50+4=3с.5д.4ед.
642=600+40+2=6с.4д.2ед.
438=400+30+8=4с.3д.8ед.
571=500+70+1=5с.7д.1ед.
634=600+30+4=6с.3д.
829=800+20+9=8с.2д.9ед.
502000008012
(502000000000+8000+12)
777000068000
(777000000000+68000)
9000055000
(9000000000+55000)
12х - 5х - 6 = 706
7х - 6 = 706
7х = 706 + 6
7х = 712
х = 712 / 7
х=101 5/7 (101 целая пять седьмых)