How many four-digit numbers can be formed with the digits 3, 5, 7, 8, 9 which are greater than 7000,
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 $=3 \times 4 \times 3 \times 2=72$