Question

how many 4digit no. can be formed if at least one digit be repeated?plz Guys give me the Sol ​

Answer 1

_ _ _ _

single digit repeat:

first from left can be filled by 9 ways

second can be filled by 10 ways

3rd by 9 ways

4th by 8 ways

so, number of numbers having single digit repeat = 10*9*9*8

2 digits repeat:

1st by 9 ways

2nd by 10 ways

3rd by 9 ways

4th by 9 ways

so, number of numbers having 2 digits repeated = 9*9*9*10

no case for 3 and 4 digits to be repeated ...

so, ur required answer is

(10*9*9*8) + (10*9*9*9)

hope it helps uh.....

(if any doubt just come to my question...)