The sum of the digits in the unit's place of all the 4 - digit numbers formed by using the numbers 3 , 4 , 5 and 6 , without repetition is :

Step-by-step solution

By fixing one digit at unit place we can form toal number without repetition is 3 ! Sum of digits =   3 !   .   6   +   5   +   4   +   3   = 6 18 = 108 ∵ n ! = n . n - 1 . . . 3 . 2 . 1