Your memories of the 5th grade are likely much more recent than mine. As I recall it, showing your work was occasionally necessary for my own edification especially when my final answer was a result of hasty guesswork predicated on faulty assumptions.
Mr Munger making it to a ripe old age and being a wealthy man to boot could literally be shaving years off your life expectancy. But I sense that if you write out all your thinking, step by step, you might find a flaw in that line of reasoning. Even though it is.. simple mathematics. ;)