How many 3-digit numbers can be formed using numerals in the set {3,2,7,9} if repetition is not allowed?

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Hi, I’m trying to find the very minimum amount it will cost me to drill from the well to the tank. I have already calculated as you can see in my picture. But I’m needing to use Lagrange I’ve already wrote my cost function and constraint. If you can help me continue I would appreciate it.