where is the concentration of salt in the water at time . Hence this states that the rate of salt loss is the concentration at time multiplied by the rate of loss of salty water volume. Now the concentration is simply the amount of salt at time divided by the total volume of water in the tank at time so:
Here you know the time taken for overflow is 20 minutes and you know the tank has total volume 50 litres so use the relationship for the volume that:
put and and then solve for .
Then substituting back into your original differential equation you should find
where is the initial amount of salt (20 kg) and is the amount of salt when overflow occurs.
I think you can take it from there, right?