Distance of the neutral axis from the top of the beam
$M = Cy$
$M = \frac{1}{2}f_c bx(d - \frac{1}{3}x)$
$80(1000^2) = \frac{1}{2}(5)(300x)(600 - \frac{1}{3}x)$
$250x^2 - 450\,000x + 80\,000\,000$
$x = 1600 \, \text{ and } \, 200$
Use $x = 200 ~ \text{mm}$ answer
Required steel area
$Q_{\text{above NA}} = Q_{\text{below NA}}$
$300x(\frac{1}{2}x) = nA_s(d - x)$
$150x^2 = nA_s(d - x)$
$150(200^2) = 9A_s(600 - 200)$
$A_s = 1666.67 ~ \text{mm}^2$ answer
Stress developed in the steel
$\dfrac{f_s/n}{d - x} = \dfrac{f_c}{x}$
$\dfrac{f_s/9}{600 - 200} = \dfrac{5}{200}$
$f_s = 90 ~ \text{MPa}$ answer
