Middling Totals in Football

[J_Treez]

This is a question about sports betting, but it's the probability aspect I'm interested in. So, I hope nobody's shorts get in a knot over it being posted here.

I'm reading Amarillo Slim in a World Full of Fat People the other day, and in the Doyle,Sailor and Slim Chapt. he goes into a discussion about middling totals in college football. He calls two bookies, and one has the total on an Alabama-Tennessee game at 45.5, whereas another has the same total at 50. So, he puts $800 on the under at 50 and another $800 on the over at 45.5. So he's risking the $80 vig for 4 numbers (46,47,48,49) to win $1600, and another number (50) to win $800. So, he's getting 20-1 on that $80 vig, and 10-1 if one bets a push at 50.

Now, HERE'S what I don't understand how to calculate. He says the TRUE ODDS of the total landing on one of those five numbers is 6-1. How the hell do you figure that?