The manometer shown below is used to measure the difference in water level between the two tanks. Calculate this difference for the conditions indicated.

Explanation

The invertd `U` tube manometer is used for finding the difference of water level in two tanks. Drawn a horizontal straight line passing through the lower level and upper level liquid in the manometer as shown in figure. Starting from top of tank `2` and moving through the tube and reaching the top of tank `1`, the manometric equation can be written as
`p_(a)+h_(1)rhog-40rho-1g+40pg=p_(a)=h_(2)rhog`
`h_(2)rhog-h_(1)rhog=40 rhog-40rho_(1)g`

But we are given
`rho_(1)=0.9rho`
`(h_(2)-h_(1))rhog=40 rhog-36rhog`
`h_(2)-h_(1)=4cm`