Question

The sum of the digit in the unit’s place of four digit numbers formed using digits $3,4,5,6$ taken all at a time is

Answer 1

If we fix $3$ in the unit place, Number of 4 digit numbers possible are $3\times 2\times 1.$
Similarly, if $4,5,$ and $6$ are fixed in units place, Number of 4 digit numbers possible are $3\times 2\times 1.$
$\therefore$ Required sum of digits in units place is $=(3+4+5+6)(3\times 2\times 1)=18\times 6=108$