Here, for water column in one arm of U-tube :
h1=10.0cm
ρ1=1 g cm−3
For spirit column in other arm of U-tube,
h2=12.5 cm
ρ2=?
If P1‌and‌P2 be the pressures exerted by water column and spirit column respectively,
Then‌P1=h1ρ1g...(i)
and‌P2=h2ρ2g...(ii)
Also as the mercury columns in the two arms of U-tube are at the same level, thus in equilibrium,
P1=P2 or h1ρ1g=h2ρ2g
Now specific gravity of spirit
=density of spirit density of water
=

0.8 g cm−3

1 g cm−3

=0.800.