How do you find Sale tax?!

Which geometric principle is used to justify the construction below?

Darragh has a golden eagle coin in his collection with a mass of 13.551\,\text{g}13.551g. an uncirculated golden eagle coin has a mass of 13.714\,\text{g}13.714g.

There are 10 chicks on ginger's farm .she has 2 chicks in one outdoor pen and 5 chicks in another. the rest of the chicks are in the barn.what is the fraction of total chicks in outdoor pens?