Permutation/Combination help!

swgd23

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Joined
Oct 24, 2010
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I have been trying to figure these out all day if someone could help me thatd be great!! I got the first part down but the second part is really bothering me:

A certain code is a sequence of 6 digits of which any can be repeated. What is the probability of guessing a given code?
- so the answer is 6! = 720, so the probability is 1/720

But this part is really bothering me:

A)Repeat problem 3. for 3 digits and 3 letters. Assume any digit or letter can be repeated in the sequence.
B). Repeat problem 4. if the digits and letters cannot be repeated.

I first used the combination rule but my work has showed me that it cancels each other out from the 3C3 rule leaving me with 1.
 
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