I need to figure out how to format the cells to keep track of customers who are using layaway. Currently we are just using a book and calculator.

I don't get how to use the formulas. Help!
I've also started a worksheet to keep track of customers' purchases, phone numbers, and bike models with serial numbers. a LOT of kids have been getting their bikes stolen, and asked me if I have the serial numbers, so I want to be able to have the serial numbers on hand to help them out in case the worst happens.