Chemical Kinetics

In a first order reaction if concentartion of reactant drops from $0.8 \mathrm{~mol} \mathrm{~L}^{-1}$ to $0.4 \mathrm{~mol} \mathrm{~L}^{-1}$ in 15 minute. What is the time required to drop concentration from $0.1 \mathrm{~mol} \mathrm{~L}^{-1}$ to 0.025 mol $\mathrm{L}^{-1}$.