Rank the pressure from highest to lowest (a) bottom of a 20-cm tall container of salt water, (b) bottom of a 20 cm-tall container of fresh water, and (c) bottom of a 5-cm tall container of mercury

The answer is C, A, B in that order.

Pressure = Density * gravity * height.

Let us look at these three substances and assume gravity is 9.8 m/s squared.

The Salt water has a density of 1029kg/m3, height of 0.2 meters.

Pressure of salt water will be 1029*9.8*0,2 = 2,016 p

Fresh water has a density of 1000kg/m3

Pressure of salt water will be 1000*9.8*0.2 = 1,960 pa

Mercury has density of 13,534kg/m3

Pressure of mercury will be 13534*9.8*0.05 = 6,631 pa

So mercury exerts the highest pressure even at only 5cm height.