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 $0.221 \,cm .$