Now from strain diagram,

$\frac{a}{X_u}=\frac{0.002}{0.0035} \\ a=\frac{4}{7}X_u$

Area :-

• Rectangle :

  $=\frac{3}{7}X_u{\times}0.446f_ck \\ A_1=0.191f_ckX_u$

• Parabola :

  $\frac{2}{3}{\times}\frac{4}{7}{\times}X_u{\times}0.446f_ck \\ A_2=0.169f_ckX_u$

• Y top

  $\frac{3}{7}X_u{\times}\frac{1}{2} \\ =\frac{3}{14}X_u \\ =\frac{3}{14}X_u+\frac{3}{7}X_u \\ =\frac{9}{14}X_u$

  $A=0.191f_ckX_u+0.169f_ckx_u \\ A=0.36f_ckX_u$

$\overline{y}top=\frac{A_1y_1+A_2y_2}{A_1+A_2} \\ \overline{y}top=\frac{\Big(0.1914{\times}f_ckX_u{\times}\frac{3}{14}X_u\Big)+\Big(0.169{\times}f_ck{\times}X_u{\times}\frac{9}{14}X_u\Big)}{0.36f_ukX_u} \\ \overline{y}=0.42X_u$