Because you are reflection 180 degrees over the x axis, the y-axis is going to stay the same (because you are not moving the square left or right. When reflecting over axis always add the same amount you took away from the other side, if that makes any sense.
The initial amount of water was 2.5 million liters Then 2 million liters remained after evaporation, therefore 0.5 liters evaporated from the swimming pool. Thus, the percentage will be given by: 0.5/2.5 × 100 = 20% Hence the answer is 20%
