Stacy runs a hot dog stand at a local fair. On the first day she served 25 customers and made £60 profit. On the second day she served 43 customers and made £135 profit. Assuming that her profit from serving n customers forms an arithmetic sequence, find a model which approximates the profit from serving n customers.
a. Explain the significance of the common difference and the constant term in your model.
b. On the third day, Stacy served 36 customers. Use your model to estimate her profit.