Who knows what this means

[wxc0212]

I need help on understanding what this means.

(n-1)
(r-1)

Now imagine that there are only two brackets () one along the side of the letters and one on the side of the numbers. There is no line in between so it is not a fraction. I know that it is used for combination questions, but i need to know how to use them and where.

Thank you very much!

[yongcheng3315]

It is the same as C(n-1, r-1).

It is the solution to the following problem:

How many positive integer solutions to the following equation?

x1 + x2 + x3+...+xr = n

An example to use C(n,r) is as this:

There are 5 people in your family. You have two movie tickets. How many ways are there to select two people from your family to go to the movie?  You do this: C(5,2) and C(5,2) = (5*4)/(2*1) = 10. This means that you have 10 ways.

[wxc0212]

Thanks alot!