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The three-letter words are of these types VVV, VCV, CVV
Case 1: VVV
Choose the first vowel any of 5 ways
Choose the second vowel any of 5 ways
Choose the third vowel any of 5 ways.
That's 5*5*5 = 125 ways.
Case 2: VCV
Choose the first vowel any of 5 ways
Choose the consonant any of 7 ways
Choose the remaining vowel any of 5 ways.
That's 5*7*5 = 175 ways.
Case 3: CVV
Choose the consonant any of 7 ways
Choose the first vowel any of 5 ways
Choose the remaining vowel any of 5 ways.
That's 5*5*7 = 175 ways.
Total = 125 + 175 + 175 = 475 ways.
Edwin