Question 397091
{{{C(x)=0.1x^2-0.5x+6.47}}}
To make math easier, I'm going to say {{{D(x) = 100*C(x)}}} 
so, {{{D(x)}}} will be in dollars
{{{D(x) = 10x^2 - 50x + 647}}}
In a quadratic equation the minimum occurs at {{{x =  -b/(2a)}}}
where the equation is of the form 
{{{ax^2 + bx + c}}}
{{{-b/(2a) = 50/20}}}
{{{50/20 = 5/2}}}
{{{D(x) = 10(5/2)^2 - 50*(5/2) + 647}}}
{{{D(x) = 250/4 - 250/2 + 647}}}
{{{D(x) = -250/4 + 2588/4}}}
{{{D(x) = 2338/4}}}
{{{D(x) = 584.5}}} dollars
250 bikes must be made to minimize cost at $584.50