Chances of hitting an A or K by the river with AK

[wayotogo]

I've read that the chance of hitting an A or a K by the river when you hold AK is 50% (http://www.learn-texas-holdem.com/te...babilities.htm) but how do you arrive at this number?

What is wrong with my own simple approach:
6/50 + 6/49 + 6/48 + 6/47 + 6/46 = .6255 = 62.55%

I'm not concerned with the chances of hitting more than one A or K, so that does not need to be discounted.

Also is there a good book out there that goes through these kinds of basic scenarios and numbers for the not so math adept people such as myself?