I believe the order-of-magnitude knowledge was very necessary if you used a slide rule. Since the slide rule allowed you to compute the significant digits the operator had to know the order of magnitude. I graduated from college before handheld/battery calculators. My physics/calculus class was more how to write the equation and derive an approximate answer than a specific answer. My grandfather gave be a slide rule when I was in 4th grade, 1962. Slide rule battery never runs down. Slide rule can be used to scrape ice off windshield. Slide rule doesn't mind hot and cold temperature. Slide rule also visible in direct sun light. My kids don't understand why approximations and knowing the order-of-magnitude are useful. Very useful for determining the expected cost of gas when filling up. Also useful for mowing, raking leaves, hauling dirt, sailing, flying and buying groceries. Of course I also worked for my grandfather in an orchard selling apples, no cash register just a cigar box and a coffee can for the tax, 3 cents on the dollar. If you couldn't figure the bill and the tax in your head you were useless. "Being comfortable with math is a way of life and an attitude" Mike McFadden mmcfadden(a)cmh.edu