We know for point source, \(I \propto \frac{1}{r^2}=\frac{K}{4 \pi r^2}\)
\(r_1=2 m, r_2=4 m=2 r_1 \)
\(I_0=16 \times 10^{-8} W / m ^2\)
at \(2 m\),
\(I_1=\frac{I_0}{(2)^2}=\frac{16 \times 10^{-8}}{4} \)
\(I_1=4 \times 10^{-8} \)
\( I_2=\frac{I_0}{4_2}=\frac{16 \times 10^{-8}}{16}=10^{-8} \)
\(\left|I_2-I_1\right|=3 \times 10^{-8} W / m ^2, x=3\)