A wire of volume 4.0 × 10⁻⁶ m³ is stretched to a longitudinal strain of 2.5 × 10⁻³…

Step 1. (a) $4.0 \times 10^{-6}\ \text{m}^3 \times 2.5 \times 10^{-3} \times 0.4 = 4.0 \times 10^{-9}\ \text{m}^3$.

Step 2. (b) $1 - 2 \times 0.5 = 0$: no change.

Step 3. (c) $4.0 \times 10^{-6}\ \text{m}^3 \times 2.5 \times 10^{-3} = 1.0 \times 10^{-8}\ \text{m}^3$.