You have a pie that is split into 3 slices.Their are 2 fathers and 2 sons.But you cant cut the pizza.
Answer: their are only 3 people. Because:
thier is a grandfather, The grandfathers son is 1 then the sons son
so thir is the grandfather >
and their is his son> fathers
so thir is the son of the grandfathers>
and the son of the grandfathers> son