I was told in my youth that the powers that be officially defined 1 inch as 2.54 centimeters
And that's the only thing worth remembering. If someone happens to remember more - good for them, but inch cm conversion factor is
defined, so everything else is just a derivative.
in my youth I was required to know that 12 inches made a foot and 3 feet made a yard
so the rest was easy
Easy? I was told to always use powers of 10, not some random number like 3 or a dozen