Answer
The stress that develops in the girder is $4.80\times 10^7~N/m^2$
Work Step by Step
We can find the strain in the girder by considering the change in length $\Delta L$ that would occur if the girder was not constrained:
$\frac{\Delta L}{L} = \alpha~\Delta T$
$\frac{\Delta L}{L} = (12\times 10^{-6}~K^{-1})(20~K)$
$\frac{\Delta L}{L} = 2.40\times 10^{-4}$
$Y = \frac{F/A}{\Delta L/L}$
$Y$ is Young's modulus
$F$ is the force
$A$ is the cross-sectional area
$\Delta L$ is the change in length
$L$ is the original length
Note that $\frac{F}{A}$ is the stress and $\frac{\Delta L}{L}$ is the strain.
We can find the stress $\frac{F}{A}$:
$\frac{F}{A} = Y~\frac{\Delta L}{L}$
$\frac{F}{A} = (2.0\times 10^{11}~N/m^2)~(2.40\times 10^{-4})$
$\frac{F}{A} = 4.80\times 10^7~N/m^2$
The stress that develops in the girder is $4.80\times 10^7~N/m^2$
