this for odds masters

[davepoker]

I guess you dont play magic geno.
lol
comparable to poker logic/ game theory though

[suitedaces84]

The only thing you need to answer these types of questions are:
1) how combinatorials work
2) common sense

The formula Adam gave is simply the hypergeometric distribution, which is the appropiate distribution. (This is a distribution that's tougher to memorize than derive.)

Let's say you had 7 lands in a deck of 60 and you wanted to know the probability of drawing exactly 4 lands in 7 picks without replacement.

There are 60c7 different ways to pick 7 cards from 60 (this that 'how combinatorials work' thing) of cards you can pick.

There are 7c4 different ways to pick 4 lands (without reguard to the other cards).

If you pick 4 lands there are there will be 3 non-lands. There will be 53c3 ways to pick those non-lands.

There are a total of 7c4*53c3 ways to pick your 4 lands (with reguard to non-lands).

So the probability of picking exactly 4 lands is 7c4*53c3/60c7 (note that this is the same thing Adam got).

0! is by definition equal to 1 so the equation holds.

n! =
n*[(n-1)*(n-2)*...2*1] =
n*[(n-1)!]

so...
n! = n*[(n-1)!]

plugging 1 in for n shows:
1!=1*0!
1 = 0!