h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I've tried following the whole method in the first half of this entry and I got the remainder 6, but that's with remainders all the way through so that's wrong. However I also did the plain 129/7 calculation and I got 18.4, meaning the remainder is 4, not 3, so unless this is a simple typo in the entry either I'm going wrong somewhere or (dare I say it) the entry is. Could someone put me straight on this one?

Hi Galigan,

129/7 is indeed 18.4 In fact it's 18.428571428571... to be precise

But this isn't remainder 4. It's 3/7, or remainder 3.

Does this make sense?

Icy

While we were studying modular arithmetic last year I was taught this neat trick for working out remainders from a decimal calculation:
1 do the calculation as normal
2 subtract the whole number part of the answer
3 multiply the decimal part by the original divisor

taking the above calculation it would be:
1 129/7 = 18.428714...
2 minus 18 = 0.428714...
3 mutiply by 7 = 3.0
so remainder 3

What a complicated process...

It would seem easier to just do the calculation

129/7 = 1 ten carry 5
59/7 = 8 remainder 3

Of course you need to know your times tables to do this.

Otherwise you could count up to the next multiple
i.e for 129
129 = 140-11 = 133-4 = 126 + 3 = remainder 3

Yes, it probably sounds complex, but I don't know my high value times tables - 16 and over, or to multiples in the hundreds for the lower value tables.

Back to the entry, the working out of days for specific dates is fascinating and fun. Great entry

Thanks, Yvonne.

Oh, don't tell Gavin, but I use your remainder method too! It's the easiest way with a calculator.

Icy

Oh ok, sorry, I haven't done maths with proper numbers for so long that I forgot what a remainder was, I was thinking that the number after the decimal point was it. That's what I get for taking A level maths, I can't remember the last time I did a question with actual numbers in it.

On the topic though, I did my last ever maths exam today, and now that's it, I'll never do any again. It's kind of sad actually because we've had the same class and teacher for 5 years and the same seats in the classroom. It's going to be weird now it's over.

Another way to phrase it would be "Reduce the fraction, and ignore the whole number" (though I'll admit that that only occurred to me because I'd been helping students learn fractions shortly before I went on maternity leave)

You ignore the remainders nearly all the way. The only remainder you want is to the last part, then you ignore the first number. e.g. 100/7 = 14 remainder 2. So ignore the 14 & just use the 2 to find the day.