MHB Strings of eight English letters

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The discussion focuses on calculating the number of eight-letter strings containing exactly one vowel, with letters allowed to repeat. The initial approach incorrectly calculates the total by considering multiple vowels, leading to a discrepancy in the final answer. The correct method involves selecting one of the eight positions for the vowel and then filling the remaining positions with consonants, resulting in the formula 8 times the number of ways to choose one vowel and seven consonants. Ultimately, the correct total is confirmed as 72,043,541,640. The clarification emphasizes that only one vowel should be present in the string, aligning with the problem's requirements.
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How many strings of eight English letters are there that contain exactly one vowel, if letters can be repeated?

My attempt:

Let us first find the number of strings that contain at least one 'a' and no other vowels.
Total number of strings including 'a' but excluding the other vowels $=22^8$
Number of strings without any vowels $=21^8$
So, number of strings that contain at least one 'a' and no other vowels $=22^8-21^8$

Sincere there are five vowels, we get $5(22^8-21^8)=85,265,070,875$. The book's answer is $72,043,541,640$. Where did I go wrong?
 
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This is how I would approach the problem...

Suppose the vowel is first, so we have 5 choices for the first letter, and the 21 letters for each of the remaining 7 letters, and applying the counting rule, we find:

$$N_1=5\cdot21^7$$

But now, we consider that the vowel may be anywhere, in 1 of 8 places, hence:

$$N=8\cdot N_1=72043541640$$
 
MarkFL said:
This is how I would approach the problem...

Suppose the vowel is first, so we have 5 choices for the first letter, and the 21 letters for each of the remaining 7 letters, and applying the counting rule, we find:

$$N_1=5\cdot21^7$$

But now, we consider that the vowel may be anywhere, in 1 of 8 places, hence:

$$N=8\cdot N_1=72043541640$$

Thanks.

But can't the vowel occur more than once? After all the question says "if letters can be repeated".
 
Alexmahone said:
Thanks.

But can't the vowel occur more than once? After all the question says "if letters can be repeated".

The problem states:

How many strings of eight English letters are there that contain exactly one vowel, if letters can be repeated?

I took this to mean that only one of the eight letters can be a vowel. :)
 
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