A typical batch filtration cycle consists of filtration followed by washing. One such filtration unit operating at constant pressure difference first filters a slurry during which 5 liters of filtrate is collected in 100 s. This is followed by washing, which is done for t_w seconds and uses 1 liter of wash water. Assume the following relation to be applicable between the applied pressure drop ΔP, cake thickness L at time t, and volume of liquid V collected in time t: ΔP/L = k₁ dV/dt ; L = k₂ V, if L is changing. k₁ and k₂ can be taken to be constant during filtration and washing. The wash time t_w, in seconds (up to one decimal place), is _______.