15. Since l is tangent line, then angle c is right and [tex]c=90^{0}[/tex];
a is twice bigger than [tex]20^{0}[/tex], then [tex]a=40^{0}[/tex];
[tex]b=360^{0}-180^{0}-40^{0}=140^{0}[/tex]
19. [tex]b+72.5^{0}+22.5^{0}=180^{0}[/tex],
[tex]b=85^{0}[/tex] and [tex]a=180^{0}-85^{0}=95^{0}[/tex]
20. 5×15=6×(6+x), 75=36+6x, 6x=39, x=7.5
