The general manager of a supermarket chain believes that sales of a product are influenced by the amount of space the product is allotted on shelves. If true, this would have great significance, because the more profitable items could be given more shelf space. The manager realizes that sales volume would likely increase with more space only up to a certain point. Beyond that point, sales would likely flatten and perhaps decrease (because customers are often dismayed by very large displays). To test his belief, the manager records the number of boxes of detergent sold during 1 week in 25 stores in the chain. For each store, he records the shelf space (in inches) allotted to the detergent.

a. Write the equation that represents the model.

b. Discuss how well the model fits.