Question 691424
Let {{{ C=0.004n^2-3.2n +660 }}}
then {{{ dC/dn = (2)(0.004n^1) - 3.2 = 0.008n - 3.2 }}}

Critical values occur when  {{{ dC/dn = 0 }}}
so, {{{ 0.008n - 3.2 = 0 }}}
{{{ 0.008n = 3.2 }}}
{{{ n = 3.2/0.008 = 400 }}}

Does {{{ n = 400 }}} correspond to a max or min?

Look at the second derivative:
{{{ dC/dn = 0.008n - 3.2 }}} so then {{{ d^2C/dn^2 = 0.008 }}}
which is +ve so critical point is a MINIMUM

Hence, minimum cost occurs when {{{ n = 400 }}} so minimum cost is found by putting {{{ n = 400 }}} in original equation

Giving minimum cost = 20