Cost of sales and credit purchases

Normally if we have to find inv or payable days
We calculate like this
Inv days = inventory / cost of sales ×365
Pay days = payables / cost of sales ×365
Sometimes there are credit purchases given which we can use instead of cost of sales
Now this has been a problem because most probably they use cost of sales for both inv and pay days but i have done a question in kaplan where they used credit purchases
How do we figure this out what to use ?