The answer is the x-coordinate of the maximum point, which is the vertex
when the coefficient of x² is negative.
The formula for that particular x-value of ax²+bx+c is
The formula for that particular x-value of -5x²+600x+1500 is
That's the answer, $60.
When they charge exactly $60, they make $19,500.
If the charge $1 less, $59, they will make only $19,495, $5 less.
If the charge $1 more, $61, they will also make only $19,495, $5 less.
Edwin