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answered 2013-06-23 23:05:59 +0100
You can solve this by hand and plot or by Sage and plot as follows:
ans=solve(20*x+10000==80*x,x) x0=ans[0].rhs() y0=80*x0 p3=point( (x0,y0), size=40) p9=p1+p2+p3 show(p9)