Calculating required focal length?

Caporegime
Joined
12 Mar 2004
Posts
29,954
Location
England
How do I calculate the range of focal lengths required to shoot sports in an arena? Say for example I'm in an arena 20x48 and can stand anywhere at the short side up to a few feet away from it if necessary. And assuming a 2.5 x 2.5m capture area.
 
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