Sum of all the four digit numbers

38664 is ans.....

for 3 in thousand's place possible outcomes are {3012,3021,3102,3120,3201,3210}.., if u observe one's place,ten's place and hundred's place..., set of ones place is {2,1,2,0,1,0,}, set of ten's place is {1,2,0,2,0,1} and hundred's place {0,0,1,1,2,2}....

that is every digit repeats itself twice..... this is similar when 1 and 2 are in thousand's place.... that is we can directly add all numbers of all one's place to get the unit's place of answer.... in case the sum is more that 10 take carry..... solving for ten's,hundred's place similarly we can get answer to be 38664