How are the Facebook Engagement/Reach Rates calculated?

Engagement rate 
(Total Number of Reactions, Comments Shares & Clicks / Number of fans at the time of posting) x 100 = %
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Engagement on reach rate 
(Total Number of Reactions, Comments, Shares & Clicks / Reach of the post) x 100 = %

​Reach rate 
(Reach of post / Number of fans at the time of posting) x 100 = %