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Find the value of y if the measures of the interior angles of a convex decagon are 5yº, 5yº, 8yº, 8yº, 9yº, 9yº, 9yº, 9yº, 9yº, and 9yº.​    

I added them up and got 80y. 

The sum of the interior angles of a polygon of n sides is given by (n – 2)(180^o).  

A decagon has 10 sides, so the sum of its interior angles is (8)(180^o).  

Thus,                        80y  =  1440^o   

                                     y  =  18^o  

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