A security alarm requires a four-digit code. The code can use the digits 0 - 9 and the digits cannot be repeated. Which expression can be used to determine the probability of the alarm code beginning with a number greater than 7 ?

The first digit must be 8 or 9, giving 2 choices, represented by 2P1. After that, 3 digits are chosen and arranged from the 9 remaining digits, represented by 9P3. There are 10P4 possible four-digit codes in total, so the probability is (2P1 × 9P3) / 10P4. This is choice C, the expression using permutations for the ordered digits.

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