A formula or function to calculate the sales promotion

Hi,

We are running a product distribution company. We need to find a way to calculate the quantity of the promotions of each product. Let me explain more:

For example we have a detergent named Persil. If we sell 2 cartons of Persil we will need to add 2 boxes as promotion and give 2 carton and 2 boxes to the buyer.
If we sell 3 carton promotion, it will be 3 boxes. The order is as below:

2c=3b (c=carton b=boxes)
3c=3b
4c=6b
5c=6b
6c=9b
7c=9b
8c=9b
10c=17b
11c=17b
12c=17+3
13c=17+3
14c=17+3+3
.
.
.

We need to automatically calculate the number of promotions we have to consider for each and all of our customers. Can you help me with a formula or function?

For example if a customer buys 172 cartons how many boxes his promotion will be.

i just gone through your requirement but it seams to be not in a particular order

i presume 2 box is a progression please go through attached file and pls check where i am wrong i given upto 178 box

Please go to the explanation sheet in attachment