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        a=180(n-2)/n 
multiply both sides by n 
an=180n-360 
collect the terms that contain n on the left 
an-180n=-360 
factor out an n on the left 
n(a-180)=-360 
divide both sides by a-180 
n=-360/(a-180) 
 
so I got a different answer than erik but I admit that I'm half asleep at the moment.
     
     
    
    
    
    
    
    
  
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