\(\frac{R}{R_0}=\frac{128}{1024}\)

\(\frac{R}{R_0}=\frac{1}{8}\)

\(\text { or } \quad \frac{R}{R_0}=\left(\frac{1}{2}\right)^3\)

\(\therefore n=3\)

\(3\) half lives in \(3\) minutes, \(1\) half life in \(1\) minute