The sum of all $4$ digits numbers that can be formed using digits $1,2,3$ and $4,$ when repetition of digits is not allowed, is

The correct option is D $66,60$
The fact that a number cannot start with 0 and as such totaling them all.
$4\times 3\times 2=24$ is the total number of numbers.

Frequency of digits at units place: $\frac{24}{4}=6$

Total of digits at units place $=(1+2+3+4)\times 6=60$

Total of them all $60\times (100+10+1)=60\times 111=6660$