Linear Demand Function # 2

[cassie]

The question:50 pills at a price of $47.50 and 80 pills at a price of
$32.50. Determine a linear demand function for these pills.

The answer: D(q)= -.5q + 72.50

I have tried everything and can't come up with the right answer. Is my formula wrong?
My work.

50= x + 47.50
80= x + 32.50

-1(50= x + 47.50)
-50= x + -47.50

-50= x + -47.50
80= x + 32.50
30= 32.50
x= .92

50= .92(x + 47.50)
50= .92x + 43.70

y= .92x + 6.30 is my answer

 

[cassie]

Never Mind. I figured it out.

(50, 47.50) (80, 32.50)

32.50 - 47.50
80-50 = -15/30= -.5

47.50= -.5(x + 50)
47.50= -.5x - 25
+25 +25

D(q)= -.5x + 72.5

Thank you anyways, I am sorry to have wasted your time.