How many strings of length 8 over the alphabet {X, Y, Z} have either exactly three X's or exactly four Y's?
Added by Rheeanna M.
Step 1
The remaining five characters can be either Y or Z. The number of ways to choose 3 positions for X's out of 8 is "8 choose 3" = 56. The remaining 5 positions can be filled with either Y or Z, so there are 2^5 = 32 ways to fill these positions. So, the total Show more…
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