Answer:
$29
Step-by-step explanation:
When selling at $18, profit is $8
Profut = (8 + x)(1200 - 40x)
= 9600 + 1200x - 320x - 40x²
= -40x² + 880x + 9600
= -40[x² - 2(x)(11) + 11² - 11²] + 9600
= -40(x - 11)² + 40(11)² + 9600
= -40(x - 11) + 14440
Max revenue hence max when x = 11
Selling price = 18+11 = $29