In sexual reproduction, of each chromosome in a pair has the same genes, how is genetic variety possible?

This answer will depend on if you are speaking about asexual reproduction or not.

Variety is possible here because the the product genes are half from the mother and half from the father. So a child might have blue eyes from his mother, but red hair from his father, etc. Different genetic mutations are the initial cause of all differentiation between genes, which are passed down from each generation to the next through this same process.

In Asexual reproduction, all reproductions are essentially clones of the parent, and no genetic variation, other than mutations will occur.

the density of gold = 19.3 g/cm3

the relation between density and mass and volume is

density = mass / volume

given :

mass of gold = 13 g

density = 13g / volume = 19.3

volume = 13 /19.3 = 0.67 cm^3

thus volume of gold nugget = 0.67 cm^3