The core of a transformer is built from silicon steel sheets 0.30 mm thick…

Step 1. The loss in each cubic metre. $\dfrac{P}{V} = \dfrac{\pi^2 \times (60\ \text{Hz})^2 \times (1.3\ \text{T})^2 \times (0.30 \times 10^{-3}\ \text{m})^2}{6 \times 4.7 \times 10^{-7}\ \Omega\,\text{m}} = 1916\ \text{W/m}^3$.

Step 2. The whole core. $P = 1916\ \text{W/m}^3 \times 0.020\ \text{m}^3 = 38.3\ \text{W}$.

Step 3. Thicker sheets. The loss goes as $d^2$. Twice the thickness gives $4$ times the loss: $4 \times 38.3\ \text{W} = 153\ \text{W}$.