Question

Find the sum of all four digit numbers (without repetition of digits) formed using the digit 1, 2, 3, 4, 5.

Answer 1

Hint: The first thing we must find out is the total number of possible arrangements of the 4 digits of the given 5, i.e., $ ^{5}{{P}_{4}} $ . Out of these arrangements, in one fifth of the cases 5 will be present in unit place, in other one fifth cases in the tens place, in another one fifth cases in the hundreds place, one fifth in the thousands place and not present in left one fifth cases. So, we can say that each term will appear at a certain place for one fifth of the total case. So, the sum of the unit digit = $ ^{5}{{P}_{4}}\left( 1+2+3+4+5 \right) $ , Similarly, the result of tens place will also be same just ten times of the ones place. So, add all the places of the digits to get the answer.

Complete step-by-step answer:
The first thing we must find out is the total number of possible arrangements of the 4 digits of the given 5, as the number of ways in which 4 out of the given 5 digits can be arranged is the number of 4 digit numbers formed. Therefore, the number of 4 digit numbers formed from the given 5 digits is $ ^{5}{{P}_{4}} $.
So, we are sure that each of the places of the numbers, i.e., ones place, tens place, hundreds place and thousands place have \[\dfrac{^{5}{{P}_{4}}}{5}\] number of times a digit appearing.
So, the sum of ones digit of all the numbers is: $ \dfrac{^{5}{{P}_{4}}}{5}\left( 1+2+3+4+5 \right) $
Similarly, tens digit will have the same sum, but we need to multiply the result by 10, as the place value of tens place is ten. So, the value of tens place addition is: $ \dfrac{^{5}{{P}_{4}}}{5}\left( 1+2+3+4+5 \right)\times 10 $
Similarly, the 100s place value is: $ \dfrac{^{5}{{P}_{4}}}{5}\left( 1+2+3+4+5 \right)\times 100 $ and 1000s place value is $ \dfrac{^{5}{{P}_{4}}}{5}\left( 1+2+3+4+5 \right)\times 1000. $
Now we will add the place values of all the places to get the answer.
Therefore, the sum of all the numbers satisfying the above condition is:
\[\begin{align}
  & \dfrac{^{5}{{P}_{4}}}{5}\left( 1+2+3+4+5 \right)+\dfrac{^{5}{{P}_{4}}}{5}\left( 1+2+3+4+5 \right)\times 10+ \\
 & \dfrac{^{5}{{P}_{4}}}{5}\left( 1+2+3+4+5 \right)\times 100+\dfrac{^{5}{{P}_{4}}}{5}\left( 1+2+3+4+5 \right)\times 1000 \\
\end{align}\]
 $ =\dfrac{^{5}{{P}_{4}}}{5}\times 15{{+}^{5}}{{P}_{4}}\times 15\times 10+\dfrac{^{5}{{P}_{4}}}{5}\times 15\times 100+\dfrac{^{5}{{P}_{4}}}{5}\times 15\times 1000 $
 $ =\dfrac{^{5}{{P}_{4}}}{5}\times 15\left( 1+10+100+1000 \right) $
 $ =\dfrac{^{5}{{P}_{4}}}{5}\times 15\times 1111 $
Now we know that $ ^{5}{{P}_{4}}=\dfrac{5!}{1!}=120 $ .
\[\dfrac{^{5}{{P}_{4}}}{5}\times 15\times 1111=\dfrac{120}{5}\times 15\times 1111=399960\]
Therefore, the answer to the above question is 399960.

Note: Remember if one of the digits is zero, then all the digits will not have the same cases of appearing in a given place, actually zero cannot appear at the 1000s place, as if it appears at 1000s place, the number will be a 3 digit number. So, in this case you have to fix zero at every place and get the number of times zero is appearing at each place and divide the left out cases among other digits.