Answer
Refractive index of liquid is $n=1.824$
Work Step by Step
As given in problem suppose in time $t$ distance travelled by light in vacuum is $d_{vac}=6.2km$
Also given in same time $t$ distance travelled by light in liquid is $d_{liq}=3.4km$
Suppose in time $t$ light travels a distance $d$,
so speed of light will be $v=\frac{distance}{time}=\frac{d}{t}$
speed of light in vaccume $v_{vac}=\frac{d_{vac}}{t}$
speed of light in liquid $v_{liq}=\frac{d_{liq}}{t}$
refractive index for material is defined as
$n=\frac{c}{v}$
$c$=speed of light in vacuum
$v$= speed of light in material
so $n=\frac{v_{vac}}{v_{liq}}$=$\frac{\frac{d_{vac}}{t}}{\frac{d_{liq}}{t}}$
$n=\frac{d_{vac}}{d_{liq}}$
putting $d_{vac}=6.2km$ and $d_{liq}=3.4km$
$n=\frac{6.2km}{3.4km}=1.8235\approx1.824$
