K&E slide rule with the magnifying cursor for utmost accuracy. Slide rules were a sort of reality check so you didn't wander off the path by several orders of magnitude. Now you punch in numbers and whatever comes out must be the right answer. In physics tests the sins, in descending order, were
- complete failure to grasp the concept
- grasping the concept but missing the goal by a factor of 100
- sloppy math but a realistic answer
My high school algebra teacher used to scold me. I'd skip all the refactoring and juggling terms around the = and write the answer. She would stand over me and recite '95% of the time you have the right answer but you skip steps. If the answer is wrong I don't have a clue why.'
Somewhere along the line I'd stumbled over the Trachtenberg method and gotten adept at it. Casting out the nines gave a high probability that the product was right. Teachers didn't like that very much either. Of course now if I want to buy 17 widgets at $1.32 apiece I hunt up a calculator.