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# Using only non negative numbers

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Hi guys: Can any one please explain this question? Thanks

How many solutions (using only non-negative integers) are there to the following equation ?
x1+x2+x3+x4+x5+x6+x7=20

Ans: C(26,6)=230,230
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Commented:
There are C(26,6) ways to arrange 20 x's and 6 + signs in a row
each of which will produce a different solution to the equation
Fixer of Problems
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Commented:
Ignore the letter 'x', it could be misleading.  There are 7 variables or values, x1 thru x7, that you can manipulate to get them to add up to 20.  Call them A,B,C,D,E,F, and G instead.  If A = 20, then all the rest are 0.  If any one of them = 20, then all the rest are 0.  If A = 19, then one of them = 1 and the rest are 0.  If you kept on with that pattern, you could make a chart of all the values.  I'm sure there is a more elegant way to calculate it but that is the basic idea.
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Commented:
>  If any one of them = 20, then all the rest are 0.
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+xxxxxxxxxxxxxxxxxxxx+++++
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+++xxxxxxxxxxxxxxxxxxxx+++
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> If A = 19, then one of them = 1 and the rest are 0.
xxxxxxxxxxxxxxxxxxx+x+++++
xxxxxxxxxxxxxxxxxxx++x++++
xxxxxxxxxxxxxxxxxxx+++x+++
xxxxxxxxxxxxxxxxxxx++++x++
xxxxxxxxxxxxxxxxxxx+++++x+
xxxxxxxxxxxxxxxxxxx++++++x
> If you kept on with that pattern
you would list all C(26,6) ways to arrange 20 x's and 6 +'s

Commented:
Thanks ozo. Can you please tell me why there is 7 rows group and then its going to 6 rows group

Commented:
ways to arrange 20 x's and 6 + signs in a row

That shows a good understanding of math. I'm no math expert, so it took some thought to see how it applied. But it's an excellent restatement of the problem. Nice.

Tom
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Commented:
> why there is 7 rows group and then its going to 6 rows group
I'm not sure what you mean.  there are always 7 groups of x's separated by +'s
(some of the groups may contain 0 x's, as long as the number of x's is a non-negative integer)
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Commented:
> why there is 7 rows group and then its going to 6 rows group
I think I understand what you mean now.
There are 7 examples of "If any one of them = 20, then all the rest are 0" and 6 examples of "If A = 19, then one of them = 1 and the rest are 0."

Commented:
ok thanks ozo.