Question 10

In a certain country, 10-digit telephone numbers with the following format were introduced:

Digits may be repeated.

10.1 How many possible 10-digit telephone numbers could be formed?

10.2 Certain restrictions were placed on the groups of digits:

• Area code: must be 3 digits and the first digit can NOT be 0 or 1
• Exchange code: must be 3 digits and the first and second digits can NOT be 0 or 1
• Number: must be 4 digits and the first digit MUST be a 0 or 1

10.2.1 How many valid 10-digit telephone numbers could be formed by applying the given restrictions? (3)

10.2.2 Determine the probability that any randomly chosen 10-digit telephone number would be a valid phone number. (2)

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