How do you figure out the percentage for best hand.

For instance when you're watching poker on tv and one person has AK and the other has 77 and they say that the 77 hand is %57 and the AK is %45 or something like that.

How do you figure out those percentages.

I understand pot odds and the rule of 4 and 2 to figure the odds of making your hand, but I don't understand the math behind which hand is most likely to win.

Thanks.