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Explanation:
The first digit must be even, so we have this subset {2,4} to pick from for that first digit. There are 2 choices here.
Whatever is picked for the first digit cannot be re-selected. So we have 5-1 = 4 choices left for the second digit, and then 4-1 = 3 choices for the third digit.
Overall we have 2*4*3 = 24 different passcodes.
