Earlier quoted context omitted.
It's my impression that doing basic mental arithmetic effectively tends to depend on understanding what happens to the magnitudes of numbers after various operations and making good approximations, which has very little resemblance to the brute force long multiplication/addition/division that is taught, but is teachable; yet approximations are almost never acceptable in school, despite being very useful in real life.…
The first thing you described is what is called "Fermi estimation". Basic mental arithmetic is being able to do addition, subtraction, multiplication and division in your head. That kind of arithmetic is strictly a prerequisite for Fermi estimation. The main purpose of Fermi estimation is not to make the calculation easier (although it can be used for that too), but to get answers while not having very good data. For…
4 foot is around 1.2m,so you have around 1.2 metric tons of water per square meter. Most buildings are designed for at most a few 100kg per square meter.