What to notice:
If we keep track of how many bees are in each generation, as well as how many males and females are in each generation, we notice something familiar and surprising.
Just for convenience, let's say that the single bee at the bottom of the diagram is in "Generation 1." His mother, just above him in the diagram, is in Generation 2. And so forth.
Why is it true?
Every bee has one female parent, so the total number of bees in one generation is equal to the number of female bees in their parents' generation.
Every one of those female bees also has a male parent, so:
Substituting the first two equations into the third gives us: