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Question
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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Solution
Let the telephone number be 45 abcd
| Number of ways to fill | |
| a | 8 |
| b | 7 |
| c | 6 |
| d | 5 |
Required number = 8P4 = 1680
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