Answer:
2:3
Explanation:
After Jean drinks a quarter of a liter of orange juice and replaces it with water, the proportion of water to juice is 1:3 (0.25 liters of water and 0.75 liters of juice).
When his brother does the same thing, there is another 0.25 liters of water added, making the total amount of water 0.25 + 0.25 = 0.5 liters.
The total amount of juice remains the same at 0.75 liters.
Therefore, the proportion of water to juice after the last mixing is 0.5:0.75, which simplifies to 2:3.