How many 4 digit numbers where the digits are distinct
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use permutation and combinations
_ _ _ _
first blank can be filled with 1 to 9 digits 0 cannot be used
second including 0 it can be filled with 9 digits as one digit is already used
third can be filed with 8 digits as two digits will be used in first two blanks
finally fourth will be filled with 7 digits as three digits are used in first three
9×9×8×7=4536
_ _ _ _
first blank can be filled with 1 to 9 digits 0 cannot be used
second including 0 it can be filled with 9 digits as one digit is already used
third can be filed with 8 digits as two digits will be used in first two blanks
finally fourth will be filled with 7 digits as three digits are used in first three
9×9×8×7=4536
sahana90:
have u learnt permutation
see first blank can't be 0 because u will get three digit number
example 0123
so it can be from 1to9
2nd blank can include o
but in 9 digits 1 digit will be used so8 digits +0=9 digits
third blank 8 digits because 2 digits will be used in first two blanks
fourth with 7 digits because 3 digits will be used in first three blanks
so 9×9×8×7=4536
did u understand?
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