A radio transmitter radiates 25 kW equally in all directions…

Step 1. The intensity. $I = \dfrac{P}{4\pi r^2} = \dfrac{25000\ \text{W}}{4\pi \times (8000\ \text{m})^2} = 31.1\ \mu\text{W/m}^2$.

Step 2. The electric field. $E_0 = \sqrt{\dfrac{2I}{\varepsilon_0 c}}$. In SI units, $\varepsilon_0 c = 2.655 \times 10^{-3}$. So $E_0 = \sqrt{\dfrac{2 \times 3.108 \times 10^{-5}}{2.655 \times 10^{-3}}}\ \text{V/m} = 0.153\ \text{V/m}$.

Step 3. The magnetic field. $B_0 = \dfrac{E_0}{c} = \dfrac{0.153\ \text{V/m}}{3.00 \times 10^{8}\ \text{m/s}} = 0.510\ \text{nT}$.