$\Sigma M_B = 0$
$4P_{al} + 2P_{st} = 4(20\,000)$
$4(\sigma_{al} \, A_{al}) + 2 \sigma_{st} \, A_{st} = 80\,000$
$4 \, [ \, \sigma_{al} (0.75) \, ] + 2 \, [ \, \sigma_{st} (0.5) \, ] = 80\,000$
$3\sigma_{al} + \sigma_{st} = 80\,000$ → equation (1)
$\dfrac{\delta_{st}}{2} = \dfrac{\delta_{al}}{4}$
$\delta_{st} = 0.5 \delta_{al}$
$\left( \dfrac{\sigma L}{E} \right)_{st} = 0.5 \left( \dfrac{\sigma L}{E} \right)_{al}$
$\dfrac{\sigma_{st} (3)}{29 \times 10^6} = 0.5 \left[ \dfrac{\sigma_{al} (4)}{10 \times 10^6} \right]$
$\sigma_{st} = \frac{29}{15} \sigma_{al}$ → equation (2)
From equation (1)
$3\sigma_{al} + \frac{29}{15} \sigma_{al} = 80\,000$
$\sigma_{al} = 16\,216.22 \, \text{psi}$
$\sigma_{al} = 16.22 \, \text{ ksi}$ answer
From equation (2)
$\sigma_{st} = \frac{29}{15}(16.22)$
$\sigma_{st} = 31.35 \, \text{ ksi}$ answer