Question 846674
{{{A+S=180}}}
{{{A=S-20}}}
Substituting,
{{{S-20+S=180}}}
{{{2S-20=180}}}
{{{2S=200}}}
{{{S=100}}}
{{{A=100-20=80}}}
{{{A+C=90}}}
{{{C=90-80}}}
{{{C=10}}}