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How many 6-digit telephone numbers can be constructed using the digits 0 to 9, if each number starts with 67 and no digit appears more than once?

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The correct Answer is:
1680
Out of 10 digits from 0 to 9, we have used 2 digits, namely 6 and 7.
So, the 3rd, 4th, 5th, and 6th digit may be filled in `(10-2)=8,7,6 and 5` ways respectively.
Hence, the required number of numbers `=(8xx7xx6xx5)=1680.`
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