Calculus about data

You choose a data and eventually another as ratio.
By example: Population and Surface give density of population of each state.

You get maxima, minima and average..

For a weighted average, you choose a data as weighting factor.
With our example, average of densities of each state isn't density for whole country 
but weighted by surface it is.

You get also median and 1st and 3d quartiles:
For the median, half of values are smaller and other greater.
For 1st quartile,  25% of values are smaller and 75% greater.
For 3d quartile,  75% of values are smaller and 25% greater.