1) 18+(16-х)·8=90
(16-х)·8=90-18
(16-х)·8=72
16-х=72:8
16-х=9
х=16-9
х=7
2) (4900:у-280):60=7
4900:у-280=60·7
4900:у-280=420
4900:у=420+280
4900:у=700
у=4900:700
у=700
3) (30·х-560):8=80
30·х-560=80·8
30х-560=640
30х=560+640
30х=1200
х=1200:30
х=40
4) 630:(30-у)+45=115
630:(30-у)=115-45
630:(30-у)=70
30-у=630:70
30-у=9
у=30-9
у=21