Sci Advisor HW Helper P: 11,927 The way i see it...It's more like $$53=188\left(\frac{1}{2}\right)^{\frac{40}{\tau_{\frac{1}{2}}}}$$ I dunno where that 8 comes into turn.I think they might mean counting the rays.Which initially (in the first minute) would be 188 and after 40mins it would drop to 53,which is just what my eq.stated... Daniel.