The intensity is defined as power radiated per unit area here power, $P=100\pi \mathrm{W}$, but in the question we want to calculate only for ten percent of origin it means, $P\text{'}=10\pi \mathrm{W}$, so the intensity, 
$I=\frac{P\text{'}}{4\pi d^2}=\frac{10\pi }{4\pi {\left(10\right)}^2}=0.025 \mathrm{W} {\mathrm{m}}^{-2}$.