A medication in the bloodstream decays according to the model

MATHEMATICS · Subjective · A Level · Mathematics · Exponentials and logarithms · Exponentials and logarithms

Problem

Q(t)=Q_0 e^{kt}

, where

k=-0.25\,\text{hr}^{-1}

. How many hours will pass before 90\% of the initial amount remains? Give your answer to two decimal places.