Accounts receivable/(Sales/365). This one is correct, you can write this as.
Accounts Receivable
-------------------------- x 365 = Average number of days.
Sales
Example:
1000
------ x 365 = 73, it takes 73 days on average for accounts receivable to be collected.
5000
===============Alternatively
365/ (Sales/Accounts) gives you the same answer
365 / (5000/1000) = 73 days.