Ask HN: Know any good maths heuristics?

For instance, I find the 'rule of 70' very useful - if you want to know how long a quantity will take to double at a given rate of growth, divide that rate into 70 to find how many time periods or iterations it will take. So eg. if population grows at 7% per year, then it will be double after (approx.) 70/7=7 years. Know any other good ones?