Combination or permutation: how many groups of nat. num. solve x1+x2+x3+x4=7 ?

[marc.foschi]

Calculate how many group of 4 digit natural number are able to solve this x1+x2+x3+x4=7

Solution is 120 but I don't understand the steps to solve it.

Thanks

 

[stapel]

Calculate how many group of 4 digit natural number are able to solve this x1+x2+x3+x4=7

Solution is 120 but I don't understand the steps to solve it.

What are your thoughts? What have you tried? How far have you gotten? Where are you stuck? (Also, in your book, is zero included in the "natural" numbers?)

By the way, since order doesn't matter in addition, this should tell you whether the question relates to combinations of numbers, or permutations.

 

[marc.foschi]

Hi, ok understood ! , 0 is included.

P7,3 = 10!/(7!3!) is the solution