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The exterior angles have to add to 360° (as a walk round the polygon turns you round one circle), so the number of sides must be 360/20 =18. An interior angle and an exterior angle add to 180°, so the interior angle is 160°. 18 of these total 160×18 = 2880°. Unless you meant total of interior plus exterior, that's 180×18=3240°.

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