Counting

How many 3-letter sequences can be formed if the second letter must be a vowel (A, E, I, O, or U), and the third letter must be different from the second letter?  

There are 26 choices for the first letter 

then            5 choices for the second letter  

then          25 choices for the third letter 

So             26 • 5 • 25 = 3,250  

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