Question 1138948
Revenue=75x-0.2x^2
this is a quadratic with maximum at vertex where x=-b/2a=-75/-0.4=187.5 
maximum revenue is $7031.20 for both 187 and 188 watches. 
profit is the difference between these two, or -0.2x^2+47x-1650
This is maximized for x=-47/-0.4 or 117.5 watches
f(117.5)=$1111.25

they differ because there are two different quadratics, basically.  The third graph has a difference between the two. Cost increases linearly, revenue will increase up to a point where beyond which more being sold and then decrease, either because of decrease in demand, oversupply, or other factors.
{{{graph(300,300,-10,150,-100,2000,-.2x^2+47x-1650)}}}


{{{graph(300,300,-10,220,-500,10000,75x-0.2x^2)}}}
{{{graph(300,300,-10,220,-2000,10000,75x-0.2x^2,28x+1650)}}}