3х+15=7-5х
3х+5х=-15+7
8х=-8
х=-8:8
х=-1
13-2х+5=х-3
13+5+3=2х+х
21=3х
х=21:3
х=7
4х+15-10х=5-11х
4х-10х+10х=-15+5
4х=-10
х=10:4 х=2,5
3х+15=7-5х
3х+5х=-15+7
8х=-8
х=-8:8
х=-1
13-2х+5=х-3
13+5+3=2х+х
21=3х
х=21:3
х=7
4х+15-10х=5-11х
4х-10х+10х=-15+5
4х=-10
х=10:4 х=2,5