Question 80736
If the first three digits of someone's phone number (do not include area code) are 777, How many different phone numbers could they have?
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The general counting rule is "if there are n ways to perform the 1st step
of a procedure and m ways to perform the 2nd step of the procedure, then
there are n*m ways to perform the procedure".
Your Problem:
Telephone Number 777-????
There are 10 ways to fill the 1st ?
10 ways to fill the 2nd ?
10 ways to fill the 3rd ?
10 ways to fill the 4th ?
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So, there are 1*1*1*10*10*10*10 ways to build the phone number.
There are 1*10^4 = 10,000 phone numbers that start with 777.
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Cheers,
Stan H.