Marketing expenditure x and annual profit y over three years are (3,22),(4,27),(6,36), in common units. Use linear regression to estimate year-four profit at expenditure five units (two decimals).
Fit $y=a+bx$. With $\sum x=13$, $\sum y=85$, $\sum x^2=61$ and $\sum xy=390$, $b=[3(390)-13(85)]/[3(61)-13^2]=65/14$. Then $a=(85-b\cdot13)/3=115/14$. At x = 5, $\hat y=(115+325)/14=31.4286$, giving **31.43**.
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