1.1.2 problem 2

(a)

If the first digit can’t be $0$ or $1$, we have eight choices for the first digit. The remaining six digits can be anything from $0$ to $9$. Hence, the solution is $$8\times 10^6$$

(b)

We can subtract the number of phone numbers that start with $911$ from the total number of phone numbers we found in the previous part.

If a phone number starts with $911$, it has ten choices for each of the remaining four digits.

$$8\times 10^6-10^4$$