The oscillating magnetic field in a plane electromagnetic wave is given by $B_y=5\times {10}^{-6}\sin 1000\pi \left(5x-4\times {10}^{8}t\right) \mathrm{T}$. The amplitude of electric field will be

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

Here, peak value of magnetic field, $B_0=5\times {10}^{-6} \mathrm{T}$

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

Using relation between magnetic and electric field, 

Peak value of electric field is $E_0=B_{0}v=5\times {10}^{-6}\times 8\times {10}^7=4\times {10}^2 \mathrm{V} {\mathrm{m}}^{-1}$