Question 273772
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Ignoring the scientific fact that these percentages of salt solution are not possible, and treating this purely as a mathematical exercise, note that the amount of salt in 10 L of 40% salt is 4L.<br>
You want that same amount of salt to be 50% of the final mixture; that means the final mixture must be 8 L.<br>
Since you started with 10L and ended with 8L, the amount of water evaporated was 2L.