Physics inverse square law question

Question

If you receive 145.68 Sieverts (the biological effect of ionizing radiation) of nuclear radiation at a distance of 6 meters from a radioactive barrel, then you receive ____ Sieverts at a distance of 11.5 m.

Answer 1

Intensity I is the source brightness F diluted by surface area of a sphere at the given distance, I₂ / I₁ = (F/4πr₂²) / (F/4π r₁²) = r₁² / r₂² and I₂ = (145.68) (6/11.5)² ≅ 39.7 ≈ 40 Sieverts to correct number of sig figs.