A billion Chinese childless couples are told to have a boy and stop having children once they do. How many total children do they have? Half the couples have one. A quarter have two. An eighth have three etc. Analogous to the math problem. Except this time, we know the answer. Two billion (don’t nitpick). So, the series total “2”. There are lots of similar problems that can be done in similar ways.
No further explanation needed. All comments welcome.