Jimmie Dean has a 20m by 30m garden.
Within the garden he opens up a 4m wide walkway.
The shaded areas are for plants and grass.
How many square meters is the garden excluding the walkway?
Finding the answer requires the knowledge of geometry about
how to calcuate the area of a trapezoid.
Chinese mathmaticians knew this thousands of year ago.
In a well-known textbook called Mathematics in Nine Chapters,
the formula is explained in Chapter 1, Section 27.