Type 1a supernova are the thermonuclear explosions of accreting white dwarfs that approach the chandrasekhar limit. in this problem, consider a white dwarf of mass 1.4 solar masses and radius 10^4 km.
assume the entire star become nickel-56 after the explosion. how long will it take all the nickel-56 to decay into iron-56?

0.19 m/s and 0.45 m/s

i think the answer is a, hope this : )

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.