d
\(N = N _0\left(\frac{1}{2}\right)^{ n }\)

\(\frac{ N _0}{8}= N _0\left(\frac{1}{2}\right)^{ n }\)

\(n =3\)

\(3 \text { half lives }=3 \text { days }\)

\(1 \text { half life }=1 \text { day }\)

\(5 \text { days }=5 \text { half life }\)

\(N = N _0\left(\frac{1}{2}\right)^{ n }\)

\(8 \times 10^{-3}= N _0\left(\frac{1}{2}\right)^5\)

\(N _0=256 \times 10^{-3}\,kg\)

\(N _0=256\,g\)