where X is monthly expenditure to promote sales and Y is the resulting monthly sales. We collect data monthly. In the past 6 months, we sold a total of 600 units. Our monthly sales expenditures are 23, 22, 16, 17, 27, 15 respectively. Us- ing monthly sales and expenditure data of all 6 months, we run a least-square regression to determine values of a and b, Based on these estimated values, we forecast that if we spend 25 next month, we can sell 110 units