A brass wire (1.2 m long, 1.2 mm² in section) hangs from a hook, and a steel wire (1.8 m, 0.80 mm²)…

Step 1. Stresses. Brass: $\dfrac{240\ \text{N}}{1.2 \times 10^{-6}\ \text{m}^2} = 2.0 \times 10^{8}\ \text{Pa}$. Steel: $\dfrac{240\ \text{N}}{0.80 \times 10^{-6}\ \text{m}^2} = 3.0 \times 10^{8}\ \text{Pa}$.

Step 2. Stretches. Brass: $\dfrac{240\ \text{N}}{1.2 \times 10^{-6}\ \text{m}^2} \times \dfrac{1.2\ \text{m}}{9.0 \times 10^{10}\ \text{Pa}} = 2.67\ \text{mm}$. Steel: $\dfrac{240\ \text{N}}{0.80 \times 10^{-6}\ \text{m}^2} \times \dfrac{1.8\ \text{m}}{2.0 \times 10^{11}\ \text{Pa}} = 2.70\ \text{mm}$.

Step 3. Total. $2.67\ \text{mm} + 2.70\ \text{mm} = 5.37\ \text{mm}$.