The difference between the freezing point (FP) and boiling point (BP) of benzene is 50 degrees X and 74.6 degrees C.
Therefore, we can say that 50 X = 74.6 C; divide both sides by 74.6 and you get 0.67 X = 1 C
We know that the FP of water is 5.5 C below the FP of benzene, so if the FP of benzene is considered "zero" in the X scale, then the water FP would be 5.5 C x (0.67 X / 1 C) = 3.7 degrees X below zero in the X scale. Therefore, the FP of water is -3.7 degrees X
The boiling point of benzene is considered to be 50 X degrees, and water boils at 19.9 degrees C higher than that.
19.9 C x (0.67 X / 1 C) = 13.3 X degrees higher than the BP of benzene.
Therefore the BP of water in degrees X is 50 + 13.3 = 63.3 degrees X