The level of toluene $($a flammable hydrocarbon$)$ in a storage tank may fluctuate between $10$ and $400cm$ from the top of the tank. Since it is impossible to see inside the tank, an open$-$end manometer with water or mercury as the manometer fluid is to be used to determine the toluene level. One leg of the manometer is attached to the tank $500cm$ from the top. A nitrogen blanket at atmospheric pressure is maintained over the tank contents.

Assume the manometer fluid is mercury and its level in the open arm is at the height of the point where the manometer connects to the tank.

If the manometer reading, $R,$ is $15cm,$ calculate the toluene level in the tank below the top $(h=?c).$ What manometer reading, $R(cm),$ would be observed if the manometer fluid is water? Which manometer fluid would you use, and why?