Find the sum of all 5 digit numbers formed by the digits 1,3,5,7,9 when no digit is being repeated.?

if we fix 9 at unit digit we can have 4*3*2*1=24 such numbers. similarly there will 24 numbers which will have 7 at unit digit, 24 numbers which will have 5 at unit digit and so on. So if we add all these numbers at unit digit the sum will be 24*(9+7+5+3+1)=600.
Same applies to all the positions.
So the final sum will be 600*10000+600*1000+600*100+600*10+600=6666600