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Combination
Thanks Bank Vault

Bank vault

Answer:   630
There are 9+8+7+6+5+4+3+2+1 = 45 different sets of 2 digits.
A set of 2 digits forms 2x2x2x2= 16 different combinations.
Two options when all digits are the first or second digits do not fit the initial conditions.

45 x (16-2) = 450 + 45 x 4 = 450 + 180 = 630

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