First order rate constant
\(K =\frac{\ln 2}{ t _{1 / 2}}=\frac{1}{ xt _{1 / 2}} \ln \frac{ A _{0}}{ A _{0}- A _{0} \times \frac{90}{100}}\)
\(\frac{\ln 2}{ t _{1 / 2}}=\frac{\ln 10}{ xt _{1 / 2}}\)
\(x =\frac{\ln 10}{\ln 2}=\frac{2.303}{2.303 \times 0.3010}=3.32\)