Is it possible to find 6-digit numbers with a sum of 8?

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SUMMARY

There are 1,001 distinct 6-digit numbers where the sum of their digits equals 8, including those with leading zeros. The problem was solved using combinatorial methods, specifically the "stars and bars" theorem, which allows for the distribution of indistinguishable objects (the sum of digits) into distinguishable boxes (the digit positions). Opalg provided a detailed solution that confirmed this count, demonstrating the effectiveness of combinatorial techniques in solving digit sum problems.

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  • Basic knowledge of number representation including leading zeros
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Here is this week's POTW:

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How may 6-digit numbers are there such that the sum of their digits is exactly 8? (Leading zeros, e.g. 041030 are allowed.)

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Congratulations to Opalg for his correct solution, which you can find below:
Solution from Opalg:
Suppose you have six boxes labelled A,B,C,D,E,F, and eight identical balls to distribute among the boxes. One way of doing this would be to put 4 balls into box B, 1 ball into box C and 3 balls into box E. The number of balls in each box A,...,F is then 0,4,1,0,3,0. Now combine those digits into a single number, 041030. Conversely, each 6-digit number whose digits sum to 8 gives rise to a distribution of the 8 balls into the boxes.

So the number of 6-digit numbers whose digits sum to 8 is the same as the number of ways of allocating 8 balls to 6 boxes. By a standard result in combinatorics, that number is $${6+8-1\choose6-1} = {13\choose5} = 1287.$$

Alternative solution from other:
The answer is equal to the coefficient of $x^8$ in the expansion of $(1+x+x^2+\cdots+x^9)^6$.

Since $1+x+x^2+\cdots=\dfrac{1}{1-x}$, the answer can be obtained also from the coefficient of $x^8$ in the Maclaurin series of $\displaystyle \dfrac{1}{(1-x)^6}=(1-x)^{-6}=\sum_{n=0}^{\infty} (-1)^n {-6 \choose n} x^n$.

Hence, the answer is $\displaystyle {-6 \choose 8}=\dfrac{(-6)(-7)(-8)(-9)(-10)(-11)(-12)(-13)}{8!}=1287$
 

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