SOLUTION: How many seven-digit phone numbers can be formed if the first digit cannot be 0 and a)repetition of digits is not permitted? b)if repetition of digits is permitted?

Question 85114: How many seven-digit phone numbers can be formed if the first digit cannot be 0 and
a)repetition of digits is not permitted?
b)if repetition of digits is permitted?

Found 2 solutions by stanbon, Nate:
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
How many seven-digit phone numbers can be formed if the first digit cannot be 0 and
a)repetition of digits is not permitted?
nine ways to select the 1st digit
9 ways to select the 2nd digit
8 ways to select the 3rd digit
...
4 ways to select the 7th digit
etc.
Total: 9*9P6 = 9*60480=544,320 ways
----------------
b)if repetition of digits is permitted?
nine ways to select the 1st digit
10 ways to select each of the remaining 6 digits
Total number: 9*10^6 = 9 million
==============
Cheers,
Stan H.

Answer by Nate(3500) (Show Source): You can put this solution on YOUR website!
How many seven-digit phone numbers can be formed if the first digit cannot be 0 and
_ _ _ _ _ _ _ ... To complete this .. you must know how many numbers CAN be used ...
a)repetition of digits is not permitted?
First Digit can not be 0 and another nummer -> 8
Second Digit can not be the chosen number -> 9
8 * 9 * 8 * 7 * 6 * 5 * 4
483,840
b)if repetition of digits is permitted?
8 * 10 * 10 * 10 * 10 * 10 * 10
8,000,000
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