[CholeraJoe]

In college physics, anything less than 12 times 12, I did in my head, if they were whole numbers. It’s pretty hard to multiply numbers with 3 decimals in your head. Beyond that I pulled out the trusty slide rule. Square roots and logarithms always required slide rule or a table.

[Little Bill]

I was an apprentice machinist, everything was in thousandths. To this day I have to convert .250 to 1/4.

An old timer taught me that .250 is the only number you need to know, if you want a fraction multiply or divide and then convert.