There is 5kg water in the container,the concentration of 5% brine flows into this container and the velocity is 2kg/min.At the same time, the brine flows out of the container in the velocity of 2kg/min.Question:when will the concentration of brine get to 4%?

I don't know if I put the question here properly,and
English is not my mother tongue , so I may make some mistakes in gramme, please forgive me ,and I holp you can understand it.
Thanks in advance!