Find the sum of all 5-digit numbers that can be formed using the first 5 natural numbers if the repetition of digits is not allowed.
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We have to form numbers using 1,2,3,4 and 5.
If we fix any digit then we get 4 ! permutations for it. Say, we fix 1 on ten thousand's place and start moving right fixing other digits.
Then we get 4 choices for thousand's , 3 choices for hundred's , 2 choices for ten's and 1 choice for one's place. Thus total permutations possible for 1 at ten thousand's place is 4 ∗ 3 ∗ 2 ∗ 1 = 2 4 .
Similarly 1 is repeated 24 times on thousand's place , 24 times on hundred's place, 24 times on ten's place, and 24 times at one's place. No more permutations for the digit 1 are possible.Similarly each digit has same pattern.
Sum of face values at ten thousand's place is 2 4 ∗ ( 1 + 2 + 3 + 4 + 5 ) = 3 6 0 For thousand's place also the sum of face value will be 3 6 0 and so on .
Now the sum of place values shall be → 3 6 0 ∗ 1 0 4 + 3 6 0 ∗ 1 0 3 + 3 6 0 ∗ 1 0 2 + 3 6 0 ∗ 1 0 1 + 3 6 0 = 3 9 9 9 9 6 0