Formula to SUM quantities then start over when value reached

[aprice]

I need to count inventory so that I always order a new case of foam filler when one is sold. There are 15 bags in a case. Some weeks we don't sell any bags, and some weeks we sell anywhere from 1 to all 15 bags. What I need to figure out is how to sum down a column so that every time the sum equals or exceeds 15, the spreadsheet adds 1 to the number of cases sold as if that week. I cannot figure it out. I have tried using MOD(C2,15) and variations of that, but it isn't working correctly.

The data looks like this:

Column A Week # z	Column B (I want a column showing Cases Sold)	Column C Bags Sold
1		3
2		0
3		5
4	(I want a 1 here)	9
5		0
6		2
7		5
8	(I want another 1 here)	8
9	(at this point I have 10 bags sold out of the case of 15, so the count just carries on down)	8

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Thank you for taking a look at this for me.
~Audrey

 

[Tetra201]

Try putting the following formula in cell B2 and copying it down:

=IFERROR(1/(1/(INT(SUM($C$2:$C2)/15)-INT(SUM($C$1:$C1)/15))),"")

 

[aprice]

Try putting the following formula in cell B2 and copying it down:

=IFERROR(1/(1/(INT(SUM($C$2:$C2)/15)-INT(SUM($C$1:$C1)/15))),"")

That's it EXACTLY!!! You are my hero! Thank you!
Can you explain to me what the formula says? I know the "If there's an error...don't do anything" part. What does the (1/(1/(INT... mean? Isn't that the running total formula (SUM($C$1:C1)? I think tried something similar.
Again, thank you!