Answer
The stress at the breaking point is $4.0\times 10^8~N/m^2$
Work Step by Step
$Y = \frac{F/A}{\Delta L/L}$
$Y$ 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 $\frac{F}{A}$:
$\frac{F}{A} = Y~\frac{\Delta L}{L}$
$\frac{F}{A} = (2.0\times 10^{11}~N/m^2)~(\frac{0.20}{100})$
$\frac{F}{A} = 4.0\times 10^8~N/m^2$
The stress at the breaking point is $4.0\times 10^8~N/m^2$.
