[tex]\displaystyle a).5+20x=20-10x \\ 20x+10x=20-5 \\ 30x=15 \\ x= \frac{15}{30} \\ x= \frac{1}{2} =0,5 \\ b).2x+90=4x \\ 2x-4x=-90 \\ -2x=-90 \\ x= \frac{-90}{-2 } \\ x=45 \\ c).3,5x=120-2,5x \\ 3,5x+2,5x=120 \\ 6x=120 \\ x= \frac{120}{6} \\ x=20[/tex]
