The human brain is located in the upper portion of the head with a volume that could be approximated by a cube l=20 cm on a side. Brain cells,
consisting of 10% neurons and 90% glia vary in size with dimensions ranging
from a few microns to a metre or so. Taking d=10 microns as a typical
dimension and consider a cell to be a cube with each side having that length,
estimate the number of brain cells in the human brain.

What is the things about?

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.