Summing up!

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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.


The answer is 3999960.

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1 solution

Pankaj Joshi
Feb 16, 2014

We have to form numbers using 1,2,3,4 and 5.

If we fix any digit then we get 4 ! 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 = 24 4*3*2*1 = 24 .

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 24 ( 1 + 2 + 3 + 4 + 5 ) = 360 24*(1+2+3+4+5) = 360 For thousand's place also the sum of face value will be 360 360 and so on .

Now the sum of place values shall be \rightarrow 360 10 4 + 360 10 3 + 360 10 2 + 360 10 1 + 360 = 360*{10}^4 + 360*{10}^3 + 360*{10}^2 + 360*{10}^1 + 360 = 3999960 \boxed {3999960}

Sum can also be calculated in the following way:

Sum= (120/60) X (12345+54321)= 60 X 66666= 3999960

Pushpendra Gawde - 7 years, 3 months ago

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I did it your way but to explain I had to give the above solution...

Pankaj Joshi - 7 years, 3 months ago

wow beautiful solution :)

Fashanu Willies Kamba-Gotora - 7 years, 3 months ago

brilliant!

Dhruva Rishyashringa - 7 years, 3 months ago

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