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a 7 digit phone number has a 3 digit prefix followed by 4 many different phone numbers are possible??

  • math -

    I assume 0 through 9 are allowed and repeats are possible. For example 000-0000 is allowed.

    How many prefixes?
    10*10*10 = 1,000
    how many digits
    10*10*10*10 = 10,000
    so 10^3 * 10^4 = 10^7 = 10 million
    I guess that is why they needed area codes.

  • math -

    7!=1*2*3*4*5*6*7=... (combinatoric)

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