Let's solve this step by step.

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(a) Repetition of letters is allowed

If repetition is allowed, each letter in the three-letter "word" can be chosen independently from the 7 letters $F, G, H, I, J, K, L$.

The number of choices for each position is $7$, and since there are 3 positions:

$7 \times 7 \times 7 = 7^3 = 343$

Answer for (a): $343$.

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(b) Repetition of letters is not allowed

If repetition is not allowed, the number of choices decreases as letters are used:

1. For the first position, there are $7$ choices.
2. For the second position, there are $6$ choices (since one letter has been used).
3. For the third position, there are $5$ choices (since two letters have been used).

The total number of words is:

$7 \times 6 \times 5 = 210$

Answer for (b): $210$.

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Final Answers:

(a) With repetition allowed: $343$
(b) Without repetition allowed: $210$

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Would you like a more detailed explanation or examples? 😊
Here are 5 related questions to expand your understanding:

1. How would the answers change if we had 8 letters instead of 7?
2. What if the "words" were required to have exactly one repeated letter?
3. Can you calculate the number of four-letter words from 7 letters with and without repetition?
4. How would you calculate the number of "words" if only vowels or consonants were allowed?
5. How does this compare to forming "words" using only 2 positions instead of 3?

Tip: When repetition is allowed, the calculations involve powers; when it isn't, permutations come into play!