Determine the number of its n-combinations

[Jrb599]

Consider the multiset {n*a, n*b, 1, 2 , 3,..., n+1} of size 3n + 1. Determine the number of its n-combinations.

I'm stuck on this one, any help would great.

 

[EnumaElish]

How is its size 3n+1 and not n+3?

 

[Jrb599]

Consider the case n=2

you get

(a,a,b,b 1,2,3) which gives you 7 elements

not 5, so 3n + 1 holds.

 

[Jrb599]

PRoblem solved, will this thread be deleted?