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Question
How many four-digit numbers can be formed with the digits 3, 5, 7, 8, 9 which are greater than 7000, if repetition of digits is not allowed?
Solution
Since the number has to be greater than 7000, the thousand's place can only be filled by three digits, i.e. 7, 8 and 9.
Now, the hundred's place can be filled with the remaining 4 digits as the repetition of the digits is not allowed.
The ten's place can be filled with the remaining 3 digits.
The unit's place can be filled with the remaining 2 digits.
Total numbers that can be formed = 3\[\times\]4\[\times\]3\[\times\]2 = 72
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