Q:

Combinatorics strategie for order

At the moment I have to deal a bit with Combinatorics but I have some problems with it. Let's say I have following situation:

Spend 1500 Euro to 4 people so that everyone has a multiple of 100 Euro. How many different ways are there to spend all this money to them?

I know that we have to choose without lay back and without order. If I look at such an exercise I always know if it is with or without lay back but I have great difficulty with the order. Can someone please explain to me why we have no order in this context or give me a strategie how I can easily check if the exercise is with or without order?

A:

We want to put 15 (k) balls (bills of €100) in four (n) boxes (people). It is a combination with repeation. It is with repeation because one can get multiple bills of €100, and the order doesn't matter because it doesn't matter for the final amount of money whether I give A €100 first and then B €100, or vice versa. 
Therefore the number of times is $$ {n+k-1 \choose k} = {18 \choose 15} $$

