Question 448986
{{{ C = x^2 - 30x + 335 }}}
This equation is a parabola {{{ax^2 + bx + c }}}
 with a minimum at {{{ x = -b/(2a) }}}
{{{ a = 1 }}}
{{{ b = -30 }}}
{{{ c = 335 }}}
{{{ -b/(2a) = -(-30)/(2*1) }}}
{{{ -b/(2a) = 15 }}}
The cost is a minimum at x = 15 thousand units
Here's a graph:
{{{ graph( 400, 400, -5, 30, -100, 1000, x^2 - 30x + 335 ) }}}