Arithmatic/Geomatrix Progression

For the following sequence of numbers, write down their corresponding difference equations, determine u15 and indicate the kind of sequences they represent:

A) 3000, -1500,750,-375,187..

B)50,47,44,41,38...

I understand that for these kind of problem that we can use either n= 0,1... or n=1,2...

As it is not indicated in the question, I use n=0,1,2... for both the question

After I am done, I checked the answer and realise that I am right for B but not A.

In part a, the answer given used n=1,2 hence their n15 and my n15 is different

My question is.. when do we know when to use n=0,1,2 or n=1,2,3? I thought we can use any one of it we like? Is it the question fault that it did not indicate?

Thanks in advance

ps exam in 10hrs