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Derek’s phone number, 336-7624, has the property that the three-digit prefix, 336, equals the product of the last four digits, 7 × 6 × 2 × 4. How many seven-digit phone numbers beginning with 336 have this property? Answer is 84 phone numbers.

I could only find 72 such numbers. Can someone help? 7 must be at least one of the digits. Last 4 digits could be {7,6,4,2}, {7, 6,8,1}, {7,3,8,2} Each set makes 24 possibilitles, hence a total of 24*3 = 72.
What 12 combination am I missing?

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You missed $\{ 7 , 3, 4, 4 \}$.

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  • $\begingroup$ Ah, ah - thanks! $\endgroup$ Oct 21, 2016 at 17:20

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