STEM field, and I think I have used it once in my 15-year career. I remember being proud of myself for actually using it
I’ve used trig a lot more, both in my actual job and outside (sines and cosines are pretty useful when one is doing 3D modeling). And geometry and linear algebra practically every day in my job.
But yeah, I have a whole rant about this. I think kids who aren’t STEM-bound should be studying a lot less algebra and calculus and whole lot more statistics and probability, which no one learns at school but which would be far more useful in everyday life to a non-STEM person!
The only problem is that we don’t have the teachers to make this work… none of the teachers understand it either.
When I was in high school I was fairly sure that I would never in my life need algebra or calculus. I was not wrong.
Math related: I have a quick mental conversion method for the temperature in C versus the temperature in F. (C to F, double the C, add 30. F to C, subtract 30, halve it. It’s close enough for my purposes.)
I wish I had a similar method for kph/mph, but only so I can tell how fast people are serving in Australia and France. I mean as easy as “double it and add X.” No division or multiplication involved. I think a math-magician might have an answer. Anyone?
The conversion factor for kph to mph to is 1.609344.
MPH to KPH
You see a sign of 50 mph, that’s around 75kmh. “X + (x/2)”
The actual calculation is: 50*1.609344 = 80.4672
See a sign of 100 mph: the mental formula is 100+50=150kph, and the actual formula gives us 160.9kph.
"The number plus half the number" for MPH to KPH.
Does that work?
KPH to MPH
If you see a sign allowing you to go 80kmh, that’s what you may think of as “a half and (half of a half’s half)”: Take 80, split it in half (40) and take the other half (40) and cut it in half (20), twice (10), then add it to the first half: 50mph.
80/1.609344 = 49.7
For example, the sign says 140kmh:
Half = 70
Half of the half: 35
Halved again: 17
70+17 = 87
Actual calculation: 140/1.609344 = 86mph. "Half plus (half of the half’s half)"
Tragically, the mental math calculation leaves you speeding.
As a STEM guy, for C to F, I use the exact, but almost as simple: Double it, subtract 10%, then add 32, which is really just additions and subtractions and gives an exact conversion (so instead of predicting that 1 deg C is the freezing point, it gives 33.8 deg F). To reverse it, I get lazy and just: Subtract 32, divide by 2, subtract 10%, which is less than 10% from the exact answer)
For kph to mph, I’d (after looking at the conversion factor of 1:0.62), I’d multiply by 6 and divide by 10 (i.e. slide the decimal point over 1 spot). That’ll get you within 10% of exact.
From cashiering, I’m good at estimating total dollar amounts and figuring out change quickly. Also from cashiering (late night) I can figure out percentages of my shift down, percent left to go. I have become a better judge of time speed distance from racing but that’s really more of a mental feel rather than actual calculation as done on paper. Racing also taught me to add and subtract with a base of 60 seconds. It’s not used so much now, but we used to time the races by stopwatches, now it’s done electronically. Homeowning errors taught me a lot about measuring properly. Lastly, cooking. Having an 11-year romance with a Brit forced me to learn cooking conversions between the American volume method and the U.K. weight method.
Yeah, I’d just have trouble remembering how many halves to take.
Generally, if I need something to better than 10%, I’ll plug it into a calculator, spreadsheet, website, etc. As the saying goes, “It’s close enough for government work.”
Seriously, though, in both JohnT’s approach and mine, we are drastically simplifying the math by realizing that precision and accuracy are not the same thing and that in normal day-to-day interactions, accuracy is what really matters, and the simpler the math the easier it is to achieve accuracy on the fly.
I would tell the students, “It doesn’t matter whether or not you use a specific branch of math as an adult. Studying math and solving problems builds your mind the way exercise builds your body.”
We’ve all seen movies of football players running down a field covered with tires. The players don’t complain that there won’t be tires on the field during the game. They understand that this practice will increase their nimbleness in avoiding tackles when they’re running with the ball.
In the message immediately following yours (#83), JohnT gives the official conversation factor of 1.609344 plus one method to convert both directions. Here is my method.
1 km is appropriately 5/8 (0.625) of a mile (within 1%). Therefore, like JohnT wrote, 50 mph is approximately 80 kph, and 100 kph is approximately 62.5 mph.
You said you only want to know “how fast people are serving,“ which I assume means in tennis. My search revealed that a tennis serve is typically 125-140 mph. Therefore, rounding further, anywhere in world except the USA, Myanmar, or Liberia, a typical tennis serve is measured as 200-240 kph.
MPH and KM/H are instinctual for me now, but if I were to try to think about it rather than go by instinct, 1.6 doesn’t seem all that hard. It’s difficult to attest to my accuracy, though, because it really, really is just instinctual or memorized.
I remember an old engineer ragging on me because I’d always look up how many feet in a mile when I needed to do the conversion. Said he could convert acre-feet to both gallons and square feet without checking a reference. I was suitably impressed.
(For miles to feet, all you need is a small dictionary.)
