process systems analysis and control 3rd edition, chapter 4, 4.17 question

A simple mixing process has primary properties. An initial 200 gal tank in normal condition is shown in Figure P4-17. At t=0, the input flow rate changed from a 5% salt solution to water. How much should the input flow be reduced after 30 minutes in order for the outlet flow to be less than 0.5%? Figure: Salt water with a concentration of 5%, volume = 200 gal

4.17. 1 . 200gal P4-17 . t=0 5% . 30 0.5%