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There are 10 possible numbers that can be chosen. Since the first digit can't be 0, there are 9 selections for the first digit. The second number can include a 0, but can't include the first number chosen, so there are 9 selections for the second digit. Then there are 8 digits for the next number, 7 digits for the next number, and so on. Total telephone numbers are: 9*9*8*7*6*5*4 = 544320 phone numbers.
5 digit telephone numbers having at least one of their digits repeated is = total possible 5 digit telephone numbers - 5 digit telephone numbers without any digit being repeated. =(10*10*10*10*10)-(10*9*8*7*6) =100000-30240 =69760
There is no single digit number greater than 9. After 9 is 10, which is a double-digit number.
The number of possibilities is 9 x 10 x 10 x 10 x 10 x 10 x 9 = 8,100,000.
The number in the units digit of the number 921 is 9.