Simple Calculus I question

[wiredspider]

Originally posted by: unsped
woops here is the right answer i forgot to turn P into TR

(d) 55
when x=50

TR = P*Q
TR = 80x-.5x^2
MR = 80-x

C = 100+30x
MC = 30

set MR = MC

80-x = 30
x = 50 (quanitity)

price = 80-25x = 55

Yep, the answer should be 55 and now it makes sense to me...