The oscillating magnetic field in a plane electromagnetic wave is...

Given here, ${B}_{y} = 5 \times {10}^{- 6}\sin 1000\pi\left(5x - 4 \times {10}^{8}t\right)T$

Here, peak value of magnetic field, ${B}_{0} = 5 \times {10}^{- 6}T$

Now the speed of the wave $v = \dfrac{\omega}{k} = \dfrac{1000\pi \times 4 \times {10}^{8}}{1000\pi \times 5} = 8 \times {10}^{7}m{s}^{- 1}$

Using relation between magnetic and electric field,

Peak value of electric field is ${E}_{0} = {B}_{0}v = 5 \times {10}^{- 6} \times 8 \times {10}^{7} = 4 \times {10}^{2}V{m}^{- 1}$