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| Math Central | Quandaries & Queries |
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Question from Miranda, a student: Account numbers for the Central Oil Company consist of seven digits. If the first digit cannot be 0, how many account numbers are possible? Thank you so much!!! |
Hi Miranda,
Think about trying to write down all the account numbers in the following way.
Write down all the possible first digits. There are 9 since the first digit can't be 0.
You can now extend each of these one digit numbers to a two digit number using any of the ten digits as the second digit. Thus there are 9 
10 = 90 possible two digit numbers.
You can now extend each of these two digit numbers to a three digit number using any of the ten digits as the third digit. Thus there are 9 10 10 = 900 possible three digit numbers.
Can you complete it now?
Penny
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