If two litres of water is evaporated from a 5 litre solution containing 3% salt, then what is the percentage of salt in the remaining solution?

Initial volume of the solution = 5 litres

Salt concentration in the solution = 3%

Two litres of water evaporated, meaning the new volume of the solution =5−2=3 litres.

When water evaporates, the quantity of salt remains the same, but the total volume of the solution decreases. Therefore, the percentage of salt in the remaining solution will increase because the same amount of salt is now dissolved in a smaller volume.

​$$\text{Percentage of salt in the remaining solution} = \frac{\text{Amount of salt}}{\text{Remaining volume of solution}} \times 100$$​​

The initial salt concentration is 3%, which means:

​$$\text{Amount of salt} = 3\% \text{ of 5 litres} = \frac{3}{100} \times 5 = 0.15 \, \text{litres of salt}$$​​

The remaining solution has a volume of 3 litres, and the amount of salt is still 0.15 litres (since only water evaporated, not the salt).

So, the new percentage of salt is:

​$$\text{New percentage of salt} = \frac{0.15}{3} \times 100 = 5\%$$​​

Thus, the percentage of salt in the remaining solution is 5%.