"Companies count on managers to make smart team decisions and oversee group members in order to maintain deadlines and keep operations running smoothly. Be prepared to answer several situational questions that will evaluate your time management skills, problem solving abilities, and ease in leading a team. Employers will also expect you to be well-versed in products and operations, so make sure to brush up on the department's relevant work."
If a customer asked you how much paint we sell in a year and you had to calculate it on a napkin during a meeting, how would you do that?
I would take the gross sales (no discounts nor returns) and divide by the average price of paint. Of course you could discuss volume/mix considerations but this would not be back of the napkin. Hope this helps.
I'd dip (dunk) that napkin in my glass of wine and ask the interviewer if that was close enough to the truth...