Question 1012809

A pentagon has side angles of 112,93,116,110. Find the missing angle measurement.

First, I added 112+93+116+110 = 431, afterwards I multiplied 431 * 180 = 77,580. Would that be correct? 
<pre>Why would you multiply the sum of 4 of the 5 interior angles by 180? Who told you to do that? Do you have some sort of resource
(text book, internet, etc.) that will give you some idea of how to approach and determine the answer to this math problem?