At first, the mercury is at the same level in both the limbs. When water is poured, suppose the fall in the mercury level in the narrow limb is x and the corresponding rise in the mercury level in the wider limb is y.
If the cross-sectional area of the narrow limb is A, then, according to the problem, the cross-sectional area of the wider limb will be 3A.
From Figure we see,
volume of tha part A.C = volume of the part FB
or, `x xxA=yxx3A` or, x = 3y
Now, pressure at point C = pressure at point D
or, `ECxx1xxg=FDxx13.6xxg`
or, `(40+x)=(x+y)xx13.6`
or, `40+3y=(3y+y)xx13.6or,(13.6xx4-3)y=40`
or, `y=40/(54.4-3)=0.78cm`.