Dr. Lizardo writes down a sequence of n letters, where each letter is A, B, or C, and the letter A appears an even number of times. How many different sequences could Dr. Lizardo have written down?
This is an active homework problem. You are not supposed to ask questions of outside forums as the page where this problem is found says "This page is copyrighted material. You can view and print this page for your own use, but you cannot share the contents of this page with others." You are breaking the rules. Please refrain from posting these problem on outside forums and use the message board instead.
