A tank initially contains litres of salt water with salt concentration grams per litre. Fresh water is pumped into the tank at a rate of litre per second. At the same time, the mixture in the tank is leaking at litres per second through a hole in the bottom of the tank. What is the concentration of the salt water in the tank, in grams per litre, after minutes?

Note: This type of question is called Hybrid. In the final exam there will be no uploading options until you have finished the exam. Once you submit the exam, a QR-code will be promoted in the screen; proceed to scan the QR-code with your mobile, and then proceed to take pictures of your hybrid questions, and upload them. 

Please be aware of this difference between the mock exam and the actual final exam.