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Comment by nickcw | Hacker News Reader

I always approximate the number of kilometers in a mile to phi (which it is pretty nearly - it is about 5% out), then you can use the Fibonacci series to convert between them.

1 1 2 3 5 8 13 21 ...

So if you want to know what 5 km is, find 5 in the Fibonacci series and the number of miles is the one just below - 3 in this case. Just use the next higher for miles -> km, eg 5 miles is 8 km.

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It's easier for arbitrary values to multiply by 1.6 imho. That's easy. Take the original value and divide by two. Then right-shift the original value by one decimal place. Add the three and there's your answer. To go the other way, multiply by 0.6 (add the half and tenth values above).

Example: How much is 37 miles in km? 37 + 3.7 + 18.5, or 59.2.

If you don't mind a really rough guess, multiply by 1.5.

Fibonacci approach can still be used for pretty much any number.

37 = 3 + 13 + 21

Find equivalents in series

3->5, 13->21, 21->34

Add up: 5+21+34 = 60

This isn't easier than multiplying by 1.6 at all, but it's also new to me and really awesome. Thank you (and the grandparent poster) for mentioning this!

Divide by six, multiply by ten. And vice versa. Quicker in the head when an approximation is required.

that sounds a lot harder than multiplying by 1.6

Im not sure which I appreciate more. An overengineered solution that probably cane about by some eureka type observation. Or the cold calm criticism that Ive learned to appreciate by making too many over engineered solutions

I value that criticism a lot, particularly when it follows fun thoughts like "why don't we write our own consensus? Raft isn't too bad, right?" In the words of Wally[0], the urge eventually goes away.

[0]: http://dilbert.com/strip/1995-01-28

It's a perfectly well-defined number system: https://en.wikipedia.org/wiki/Zeckendorf%27s_theorem

You're both upvoted. I'm going to try the Fibonacci version for a while.

(Though perhaps as is suggested with languages, it's better to start applying the new system directly, instead of performing a conversion step first.)

Yes! What is a meter? One Yard! 1km = 10 football fields (no end-zones)

I find that meters are better for thinking about walking measures, why does our system even have a yard? Think in units, metric is good. I learned this in American Public School. I was very lucky.

Memorizing a sequence of ten numbers (or so) sounds way easier than multiplying by 1.6 (or .62) without whipping out a calculator.

It's not that hard. And if it is, add 50% and a fudge factor.

If you think of 1.6 as 1 + 1/2 + 1/10, then multiplying by it is pretty easy. For example, 34 x 1.6 = 34 + 17 + 3.4 = 54.4.

The exact conversion is 1.609344, so to two places it is 1.61. That's only slightly harder than 1.6. Think of it is 1 + 1/2 + 1/10 + 1/100. So for my prior example, 34 x 1.61 = 34 + 17 + 3.4 + 0.34 = 54.74.

The operations needs, taking 1/2, 1/10, and 1/100 of the original value are generally easy in your head. The hardest thing is just keeping track of the running sum as you do that.

If you want to go one step farther, you could add a -1/1000 term to bring it to 1.609 but that is probably overkill in most circumstances. Even 1.61 is probably overkill.

multiplying with 1.6180339887.. you ment?

I multiply by 1.5. Like "metric pounds" (500g) it's close enough.

In china, they use a unit (the 斤 jin) that is equal to 500g. (Now. Historically it would have been something close.)

I got a shocked reaction "you look much smaller!" from someone when I just reported my weight in pounds as weight in 斤. I guess the lesson is it's not "close enough" when people are already sensitive to a particular scale.

I think you are correct, but I think humans are not as sensitive to distance measurements. If you sprint, and have a 100 yard and 100 meter dash time... Well certainly! If you want to say we just walked 100m to the car, not so much.

Yes, close enough*

*except for when it's not, of course :-)

Loaves of bread in Japan are still sold using 斤 (kin). A standard loaf is one "bread kin" which is defined as 340g or more.

Alternatively: multiply by ten, convert to hex, and read off as a fixed point value with 4 fractional bits.

1km = 10 = 0xA = 10/16 mile = 0.625 miles (actual: 0.621371)

2.5km = 25 = 0x19 = 1 and 9/16 miles = 1.5625 miles (actual: 1.55343)

(If it isn't obvious where the 1.6 went, you probably won't regret figuring it out.)

for me, it's easier to think of it this way:

1 mi ~ 8/5 km, so v miles ~ v x 8 / 5 kms, and w kms ~ w x 5 / 8 miles.

Or, just visualize your car's speedometer: 100kph=62mph and 100mph=160kph; approximate from there.

http://a57.foxnews.com/global.fncstatic.com/static/managed/i...

I love this solution. It doesn't help you answer the question "how far is a 10K" though. If you factor into 5 and 2 and use your method on each factor, you get 3mi which is pretty wrong.

Hmm

10 km = 2 * 5 = 2 * 3 miles = 6 miles which is pretty close

The way you seem to have done it: 10 km = 1 mile * 3 miles And your units have changed so this can never be right.

Before I had it memorized, I tried with "11.66" which breaks down into two parts ("1:1.6" and "1.6:6")

1 mile = 1.6 kilometers

1.6 gallons = 6 liters

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