Probability of drawing an ace Lets say I've drawn 20 cards from a deck and The 20th was an ace. I'm trying to figure out the probability of drawing an ace in the 21st draw.
I'm sure this is just clever combinatorics but all my attemps have led nowhere. The probability of The first event is 4/52, right? And the second one is 3/32? 

There are 51 possibilities for card number 21, out of which, 3 could be aces. Hence, probability is 3/51.
Note that this probability is independent of the card numbers (20 and 21, in this case)
