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Decreasing the price in 42-32 = 10 dollars increases the demand in 5 shirts, from 5 to 10.
Hence, the slope of the straight line of the demand function is = 0.5.
Then the function itself is
d(p) = 5 - 0.5(p - 42). (1)
CHECK it : at p= 42 you have d(p) = 5. ! Correct !
at p= 32 you have d(p) = 5 - 0.5*(32-42) = 5 - 0.5*(-10) = 5 + 5 = 10. ! Correct !
You can transform the function (1) to the form
d(p) = 5 - 0.5p + 21, or d(p) = 26 - 0.5p. Thus, m = -0.5, b = 26. ANSWER
Solved.
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