A. We are going to form 7 digit numbers from the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
where the first digit cannot be 0 or 1.
so we have 8 choices for the 1. digit, and 10 choices for all the other 6 digits.
this means there are
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possible numbers.
b.
consider the numbers which start with 911. There are
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such numbers, since for the 4th, 5th, 6th and 7th digits we have 10 choices.
then we remove this number, from the one we found in a:
There are in total

numbers which don't start with 911.
Answer:
a.

b.7,990,000