a
Height of the water column, \(h_{1}=10+15=25 cm\)

Height of the spirit column, \(h_{2}=12.5+15=27.5 cm\)

Density of water, \(\rho_{1}=1 g cm ^{-3}\)

Density of spirit, \(\rho_{2}=0.8 g cm ^{-3}\)

Density of mercury \(=13.6 g cm ^{-3}\)

Let \(h\) be the difference between the levels of mercury in the two arms.

Pressure exerted by height \(h,\) of the mercury column:

\(=h \rho g =h \times 13.6 g \ldots(i)\)

Difference between the pressures exerted by water and spirit:

\(=h_{1} \rho_{1} g -h_{1} \rho_{1} g\)

\(=g(25 \times 1-27.5 \times 0.8)=3 g \ldots (i i)\)

Equating equations ( \(i\) ) and (ii), we get:

\(13.6 hg =3 g h\)

\(=0.220588 \approx 0.221 cm\)

Hence, the difference between the levels of mercury in the two arms is \(0.221 \,cm .\)