I am looking to solve the problem for a case study:

The Company sells N units of merchandise over the period of T

Some of the units are returned. For each unit I have Sale date and return date (if returned)

I need to derive a distribution which looks like this:

P1% propability that no units are returned

P2% probability that Q1% units are returned

P3% probability that Q2% units are returned

...

Pn% probability that Q(n-1)% units are returned

P1+...+Pn = 100%

Could you help me identify which distribution is that and how to go about estimating Pi and Qi? Can I use Excel for it?

Any suggestions welcome