No. 18
[tex](2x + 15) + (10x + 45) = 180[/tex]
[tex]12x + 60 = 180[/tex]
[tex]12x = 180 - 60[/tex]
[tex]12x = 120[/tex]
[tex]x = \frac{120}{12}[/tex]
[tex]\boxed{x = 10}[/tex]
.
No. 19
[tex]5x + 60 + 40 = 180[/tex]
[tex]5x + 100 = 180[/tex]
[tex]5x = 180 - 100[/tex]
[tex]5x = 80[/tex]
[tex]x = \frac{80}{5}[/tex]
[tex]\boxed{x = 16}[/tex]
.
No. 20
[tex]4x + 80 + 100 + 120 = 360[/tex]
[tex]4x + 300 = 360[/tex]
[tex]4x = 360 - 300[/tex]
[tex]4x = 60[/tex]
[tex]x = \frac{60}{4}[/tex]
[tex]\boxed{x = 15}[/tex]
