[tex]a+b+c=270\\3a=b+20\Leftrightarrow a=\frac{b+20}{3}\\
c=\frac{a+b}{2}=\frac{1}{2}\cdot(\frac{b+20}{3}+b)=\frac{1}{2}\cdot\frac{b+20+3b}{3}=\frac{4b+20}{6}\\
\frac{b+20}{3}+b+\frac{4b+20}{6}=270\\
2b+40+6b+4b+20=270\cdot6\\
12b+60=270\cdot6=620\\
12b=1620-60=1560\\
b=\frac{1560}{12}=130\\
a=\frac{b+20}{3}=\frac{150}{3}=50\\
c=\frac{2b+10}{3}=\frac{270}{3}=90[/tex]