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2 answers:
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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
possible numbers.
b.
consider the numbers which start with 911. There are
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
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
b.
consider the numbers which start with 911. There are
then we remove this number, from the one we found in a:
There are in total
Answer:
a.
b.7,990,000