TRY IT:

Passwords for your company's computer system consist of two letters (lower case) followed by six numbers.

ex:

How many possible passwords are there?

How many if no letters an no numbers can appear more than once?

One more thing:

On the password problem you just did, what if letters and numbers CAN appear more than once, BUT no two adjacent (next to each other) letters or numbers can be the same?

OK:

                            not OK: 

Let's think it out:

No restrictions yet.

Suppose the first guy is a t:

Then, this guy can't be a t...  this leaves 25 possibilities.