How to do this question? Have to use a specific Operations Management and Supply Chain Management Concept to get the answer

Shoppers flock to the NTUC FairPrice on payday to stretch their food dollars as far as they can go. Customers enter at the rate of 50 customers per hour and spend an average of 30 minutes selecting their groceries.

When it comes time to check out, shoppers have their choice of two types of checkout lines; an express line for shoppers with 20 items or fewer and a single line for shoppers regardless of how many items are in their cart.

(It is important to note that this one non-express line feeds multiple checkers - when one customer is done, the next person in the non-express line simply moves to the open register) About 25% of all shoppers can (and do) choose the express line.

There are on average three shoppers in the express line and six shoppers in the non-express line Once it is their turn, it takes express checkout shoppers an average of two minutes to check out and non-express customers an average of five minutes to check out.

i. How many customers are in the NTUC FairPrice at any given time? (25 marks)

ii. How long does each type of customer spend in the store? How long does the average customer spend in this store? (25 marks)