Question 1039269
the {{{ t }}} of the vertex ( in this case a maximum ) of a
parabola is given by the formula:
{{{ t[max] = -b/(2a) }}} when the equation has the
form {{{ a*t^2 + b*t + c }}} ( {{{c = 0 }}} in this case )
{{{ C(t) = -.0004t^2 + .08t }}}
{{{ a = -.0004 }}}
{{{ b = .08 }}}
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{{{ t[max] = -b/(2a) }}}
{{{ t[max] = -.08 / ( 2*(-.0004 )) }}}
{{{ t[max] = .08 / .0008 }}}
{{{ t[max] = 100 }}}
The maximum serum concentration is reached
in 100 min
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{{{ C(t) = -.0004t^2 + .08t }}}
{{{ C(100) = -.0004*100^2 + .08*100 }}}
{{{ C(100) = -4*10^(-4)*10^4 + 8 }}}
{{{ C(100) = -4 + 8 }}}
{{{ C(100) = 4 }}} 
The maximum concentration is 4 mg/L
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Here's the plot:
{{{ graph( 400, 400, -24, 240, -2, 6, -.0004x^2 + .08x ) }}}