$\begin{array}{1 1} 1.386 \times 10^{8}\;min \\ 0.693 \times 10^{8}\;min \\ 10^{8}\;min \\ 4.238 \times 10^{8}\;min\end{array}$

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Answer: $1.386 \times 10^{8}\;min$

This is very similar to the problems that involve calculating rate-constants for a partially completed reaction.

Out of a max thickness of 2mm, % complete when it forms an oxide layer of thickness 1.5mm $ = \large\frac{1.5}{2}$$ = 0.75 = 75\%$

$\Rightarrow$ The remaining $\% = 1 - 0.75 = 0.25 = 25\%$

$\Rightarrow ln\;A = -kt + ln\;A_0 \rightarrow ln 0.25 = -t \times 10^{-8}\;min^{-1} + ln \; 1 $

$\Rightarrow -1.386 = -t \times 10^{-8}\;min^{-1} \rightarrow t = 1.386 \times 10^{8}\;min$

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