Evaporation and Drying

In a falling rate drying period governed by Henderson-Pabis model $MR = a \exp(-k t)$, at $t = 0$, $MR = 1.0$. If $k = 0.40\text{ h}^{-1}$, the time in hours required for the moisture ratio to drop to $MR = 0.10$ is ________ (round off to 2 decimal places).