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How many six-digit telephone numbers can be formed if the first two digits are 45 and no digit can appear more than once?

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Best answer

There are total of 10 digits. Let the telephone number be 45abcd.

There are 8 digits left for the choice of a, b, c, d as repetition is not allowed.

Consider the following table:

∴ Required number of numbers formed = 8 × 7 × 6 × 5 = 1680

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