QUESTION: If the
value of g on Earth is
10 N/kg, then what
will be the weight of
a mass of 20 kg;

w = m x g  

= mg

w = 20 x 9.8

20 x 9.8 = 196

The weight of an object with a mass of 20 kg is 196 newtons.

I used the approximate gravitational acceleration but it would be 200N if you used your 10m/s acceleration.

0.55x745.7j and a calculator

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.