The grade-tonnage distribution for the ultimate pit of a mine is given below.
\[ \begin{array}{|c|c|} \hline \text{Cu grade (\%)} & \text{Cumulative million tonnes below the grade} \\ \hline 0.1 & 0 \\ 0.4 & 15.0 \\ 0.5 & 17.0 \\ 0.6 & 18.0 \\ 0.7 & 19.0 \\ 0.9 \text{ and above} & 23.0 \\ \hline \end{array} \]
The mill cut-off grade is 0.5% Cu. The annual mining capacity (ore + waste) is 4.5 million tonne and milling capacity is 1.0 million tonne. Excavation is planned in such a way that either the mine or the mill runs at full capacity throughout. The planned life of the mine, in years, is \(\underline{\hspace{1cm}}\). (round off to one decimal place)