Peter Grant had a piece about the frustrations of people who depend on computers and can’t (or won’t or don’t have time) to double-check what went into the computer and the numbers that come out. When DadRed was tutoring in the public schools, he encountered an absolute absence of the ability to estimate and round.
Now, I am not exactly innumerate, but close. Doing math in my head hurts. I do it, and do it often, but it is not easy and requires complete concentration. When that’s not possible – say, when I’m in the grocery store – I estimate.
Why this is no longer taught escapes me, but I’m sure there are modern, up-to-date pedagogical reasons. Or it’s not on the standardized tests, so no one has time to cover the material. Which is really a shame, because as my readers well know, it’s one of the most important life-skills you can have up your sleeve.
When I had a hard grocery budget of $35/week, I rounded all prices up if they were more than twenty-five cents. This allowed for sales tax. And I’d add every time I put something into the cart or basket, making certain that I was below my weekly limit. It worked very well, but it was tricky keeping a mental list of the prices of everything constantly in mind. I rarely went over budget, and those weeks usually coincided with deep-discount meat, so I could amortize the cost over two or three weeks. When stew meat hits $1/pound, you go a little wild, or at least I do when I have freezer room.
I almost always round up. It’s safer when dealing with money and budgets. Round up expenses, round down income, and you have a safe margin. Round up the stress load, round down the estimated materials strength, and your home-made scaffold is less likely to fall down with you on it. An old Boeing mechanic who had worked on the 707 and several other planes chuckled about the 707 being overbuilt. “It was the last commercial airliner designed with slide-rules, and everyone rounded up at each stage.” Made perfect sense to me. Especially for something no one has done before, allowing a little wife-and-kids-room in calculations is safer.
But, I grew up without being allowed to use a calculator until I was in college. I had to learn the multiplication tables, and one of the reasons I have trouble to this day is that I didn’t. When I changed school districts, I no longer had to learn everything cold by rote. So I didn’t, and I managed to bluff my way through until I hit algebra and bounced. It wasn’t until college and trig that I ever recovered. Trig was story problems. I can do story problems because there is a reason for them. I like my bridges to stand up, and my flight path to end up where I had intended for it to. (Ah, vectors, how I hated thee until I encountered wind correction angles!)