The credit card number is coded by subtracting each digit of the number from 9.

To code the credit card number 3201 2342 3458 0931 subtract each of the digits from 9. This gives (9-3)(9-2)(9-0)(9-1)(9-2)(9-3)(9-4)(9-2)(9-3)(9-4)(9-5)(9-8)(9-0)(9-9)(9-3)(9-1)

=> 6798 7657 6541 9068

If a credit card number is coded as 2341 0135 7923 0133 the original number is (9-2)(9-3)(9-4)(9-1)(9-0)(9-1)(9-3)(9-5)(9-7)(9-9)(9-2)(9-3)(9-0)(9-1)(9-3)(9-3)

=> 7658 9864 2076 9866

If x represents a digit of the number, f(x) = 9 - x. The domain of f(x) is [0, 9]

`f^-1(x)` = 9 - x. The domain of `f^(-1)(x)` is [0, 9]

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