1. ## basic probability

You have 130 keys and only 1 unlocks a certain door.

What are the odds you draw that key out of a the pile of 130 on your nth try?

2. Originally Posted by mathman88
You have 130 keys and only 1 unlocks a certain door.

What are the odds you draw that key out of a the pile of 130 on your nth try?
Draw a tree diagram. You will quickly see that Pr(N = n) = 1/130.

3. I wrote the question down wrong... here it is corrected:

You have 130 keys and only 1 unlocks a certain door.

What are the odds you draw that key out of a the pile of 130 BY your nth try?

4. Originally Posted by mathman88
I wrote the question down wrong... here it is corrected:

You have 130 keys and only 1 unlocks a certain door.

What are the odds you draw that key out of a the pile of 130 BY your nth try?
$\Pr(N \leq n) = \Pr(N = 1) + \Pr(N = 2) + .... + \Pr(N = n) = \cdot$