(80:х)*3=900
240:х=900
х=-240+900
х=660
6х-10x-40=14+2x
6x-10x-2x=14+40
-6x=54
X=-9
5+x=3x-12x-15
X-3x+12x=-15-5
10x=-20
X=-2
5x+20=8x-10
5x-8x=-10-20
-3x=-30
X=10
X+2=5x-2x+6
X-5x+2x=6-2
-2x=4
X=-2
3-3x-2+x=2
-3x+x=2+2-3
-2x=1
X=-0.5
(5+5)-(5+5)+(5-5)+(5+5)=10