Consider a tank used in certain hydrodynamic experiments. After one experiment the tank contains 250 L of a dye solution with a concentration of 5 g/L. To prepare for the next experiment, the tank is to be rinsed with fresh water flowing in at a rate of 5 L/min, the well-stirred solution flowing out at the same rate.

Find the time t1 that will elapse before the concentration of dye in the tank reaches 2% of its original value