Question 1098121
Yes, your approach is fine.
-b/2a is the approach to get the vertex of the minimum, and it does equal 20, or 20,000 phones.
5(20^2)-200(20)+4000=2000-4000+4000=$2000
{{{graph(300,300,-10,30,-1000,10000,5x^2-200x+4000)}}}