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Since the numbers begin with 123, they all contain a 3. As a result, they all "contain 3 or 7".

So the remaining 4 digits can be any digits and this gives 104 = 10,000 such numbers.

Q: How many 7 digits telephone numbers are there that begin with 123 and that contain 3 or 7?

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Credit cards have a series of 14-16 digits, beginning with the numbers 3, 4, 5 or 6. American Express cards begin with 3 and have 15 digits. Diners Club and Carte Blanche (JCB) cards begin with 3 and have 14 or 16 digits. Visa cards begin with 4 and have 16 digits. (Old cards used to have 14 digits). Mastercard cards begin with 5 and have 16 digits. Discover cards begin with 6 (usually 6011) and have 16 digits.

because the switches were hard-wired together and fairly hard to re-wire telephone exchange buildings in many larger cities in north America were dedicated circuits that begin with the first two or three digits of the standard 7 to the phone numbers .

Right because you start with the number that begins

24, of which 6 will begin with zero. Excluding these, gives 18.

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