Answer
$4$
Work Step by Step
D only occurs once, therefore we choose where D will be, that is 4 choices, and then the other letters are R's.
So there are $4$ four-letter sequences that contain only letters R and D with D occurring only once.
Finite Math and Applied Calculus (6th Edition)
by Waner, Stefan; Costenoble, Steven
Published by
Brooks Cole
ISBN 10:
1133607705
ISBN 13:
978-1-13360-770-0
Chapter 6 - Section 6.3 - Decision Algorithms: The Addition and Multiplication Principles - Exercises - Page 426: 49a
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