What’s the least number of chairs you would need around a table to sit four fathers, two grandfathers, and four sons?
Four. For two grandfathers and two fathers.
Two grandfathers – each grandfather is also a son and a father; this makes 2 grandfathers, 2 fathers and 2 sons.
Two fathers – each father is also a son; this makes 2 fathers and 2 sons.
In total, it’s 2 grandfathers, 4 (2+2) fathers and 4 (2+2) sons.