Question

How many 4- digit numbers are there with no digit is repeated

Answer 1

First digit 1 of 9 (excludes 0 thus 1,2,3,4,5,6,7,8,9 = 9 options.
Second digit 1 of 9 (includes 0) thus 0 plus 8 of the remaining 9 = 9 options.
Third digit 1 of 8 total of 10 digits minus the 2 already used = 8 options.
Fourth digit 1 of 7 total of 10 digits minus the 3 already used = 7 options.
9*9*8*7=4,536 different numbers.

Answer 2

Repeated Question, already asked

see the link here:  http://www.sarthaks.com/1464/how-many-4-digit-numbers-are-there-with-no-digit-repeated