Step-by-step explanation:
We find the stationary point of (20 + 2x)(1200 - 60x),
where x is the unit rate of increase/decrease.
d/dx [(20 + 2x)(1200 - 60x)]
= d/dx (-120x² + 1200x + 24000)
= -240x + 1200.
When d/dx [(20 + 2x)(1200 - 60x)] = 0,
We have -240x + 1200 = 0. => x = 5.
Price per shirt that maximise revenue
= $20 + $2x = $20 + $2(5) = $30.
Maximum revenue
= (20 + 2(5))(1200 - 60(5))
= $30 * 900 = $27,000.
