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 ?