Suppose that a maximum of 25 gm of salt dissolves in 100 gm of water. Any more salt, if added, remains undissolved and a sediment falls at the bottom. Now, water is evaporated from 1 kg of a 4% solution at the rate of 28 gm per hour. After how long will it start sedimenting approximately?

Since 25 gm of salt can be dissolved in 100 gm of water, 1 gm of salt can be dissolved in 4 gm of water.............. (1)

It is given that the solution of 1000 gm has 4% salt, which means 40 gm of salt and 960 gm of water. 

For 40 gm of salt, we need 160 gm of water (from statement 1). If the amount of water goes down further, there will be sedimentation. 

Hence, sedimentation shall start right after 800 gm of water has been evaporated (since anything below 160 gm would result in sedimentation).

Time taken to evaporate 800 gm of water is $$\ \frac{\ 800}{28}$$ or approximately 29 hours.