Before counting quadrilaterals, we'll first start by counting smallest squares or unit squares, the size of a unit square is \(1\times 1\).
In any regular grid, the number of unit squares is \[a\times b\].
(This is quite easy to prove)
In the diagram above, how many unit squares are there in the grid?
In any regular grid, the number of unit squares is \(a\times b\), here, \(a=3\), \(b=5\), \(a\times b=15\), so there are 15 unit squares in the grid.
This is one part of Grids and Quadrilaterals.