$\delta_{st} = \delta_{al} = \delta$
$\left( \dfrac{\sigma \, L}{E} \right)_{st} = \left( \dfrac{\sigma \, L}{E} \right)_{al}$
$\dfrac{\sigma_{st} \, (10)}{29 \times 10^6} = \dfrac{\sigma_{al} \, (15)}{10 \times 10^6}$
$\sigma_{st} = 4.35 \, \sigma_{al}$
When σal = 10 ksi
$\sigma_{st} = 4.35(10)$
$\sigma_{st} = 43.5 \, \text{ ksi } > 18 \, \text{ ksi}$ (not okay!)
When σst = 18 ksi
$18 = 4.35 \, \sigma_{al}$
$\sigma_{al} = 4.14 \, \text{ ksi } okay!)
Use σal = 4.14 ksi and σst = 18 ksi
$\Sigma F_H = 0$
$P = R_1 + R_2$
$P = \sigma_{al} \, A_{al} + \sigma_{st} \, A_{st}$
$P = 4.14(1.25) + 18(2.0)$
$P = 41.17 \, \text{ kips}$ answer
