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Question
Answer the following:
How many six-digit telephone numbers can be formed if the first two digits are 45 and no digit can appear more than once?
Solution
Digits are 0, 1, 2, ... , 9 i.e., n = 10
The first two digits in each telephone number are 45.
∴ out of six-digit telephone numbers, only 4 digits from the remaining (10 – 2) = 8 digits are to be arranged.
Hence, the number of ways in which 6-digit telephone numbers can be formed
= 8P4
= 8 × 7 × 6 × 5
= 1680
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