A student attaches a block of mass M to a vertical spring so that the block-spring system will oscillate if the block-spring system is released from rest at a vertical position that is not the system’s equilibrium position. The figure shows three positions of the spring as it travels from point X to point Y and then from point Y to point Z. Point X is the lowest point of the center of mass of the block as it oscillates. Point Y is the equilibrium position of the block-spring system. Point Z is the highest point of the center of mass of the block as it oscillates. The gravitational potential energy of the block-spring-Earth system is considered to be zero at point X.

the most reliable source of information.is wikipedia

u=13

v=30

s/t=u+1\2at

=13+431/2

=13+265.5

=278.5

i think the answer would be reserve.

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.