It has been a while since I was out of school so I am hazy on stuff like factorials or binomial coefficients. So I need an explanation on how to use them for me problem.

I want to join a lottery where I have to pick $4$ numbers out of the numbers $1$ to $39$. The order of these picked numbers does not matter. To keep it easy on myself to check results, I always use the number $16$ as one number. I know this reduces my odds of winning the lottery, but I don't know how much this reduces it as opposed to picking all $4$ numbers at random. (I guess we can assume that all $39$ numbers have the same odds of getting picked.)

Any ideas would be much appreciated.