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Comment by iherbig | Hacker News Reader

No discernible pattern and the numbers very quickly get large, I imagine.

2 is always even. 3 only gets to 30 after 3 x 10. 4 has a quick pattern (4>8>2>6>0>..). 5 always ends in a 5 or a 0.

7's pattern is long 7>4>1>8>5>2>9>6>3>0>.. with no sense to it and the numbers get big fast.

8's pattern is short (8>6>4>2>0>..) but the numbers are unwieldy getting into the 40s really quickly.

9's pattern is easy: start with 09. Add one to tens-place and subtract one from ones-place. So it's long, but pretty: 9>8>7>6>5>4>3>2>1>0>..

I skipped 6 because it seems to me that 6 would probably be pretty difficult for the same reason 8 is difficult. Its pattern is short (6>2>8>4>0>..) but there's less sense to it than there is to 8's pattern, and the numbers get large rather quickly.

At least, that's the reasoning I've always figured. I have no idea.

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Just one idea: you don't come across things in multiples of 7 that often.

UK: eggs come in 6s. Pizzas get cut into 4s and perhaps 8s for a really huge one, so multiples of those numbers get seen around. Half is sort of easy (all my adult students can usually find half of a figure, especially if I encourage them to think of paying it out in money with £10 notes and £1 coins). 5 fits into the decimal system nicely. Threes are easy to count up.

> Just one idea: you don't come across things in multiples of 7 that often.

Days of the week?

You got me there!

But just thinking around weeks, apart from a fortnight do you actually count the days in multiples of 7 or just think about months or a period of weeks?

I became good at sevens working in a pharmacy store room in a large hospital where we did continue to count in weeks. Boxes of 112 were common. Inside you would find blister packs arranged in rows of seven with the days of the week printed.

Nice example!

> Just one idea: you don't come across things in multiples of 7 that often.

American football scores.

I'm in the UK. I'll read up on those - US children should be better at their times tables than others then!

http://www.theguardian.com/news/datablog/2013/may/31/times-t...

This is why I never had problems with multiples of 7. 8's on the otherhand can still give me pause...

Some additions:

3's multiples' digits always add up to a multiple of three. So 69 is 6+9 which is 18, a multiple of three.

9's multiples' digits always add up to a multiple of nine. So 108 is 1+0+8 which is 9, a multiple of 9.

6's multiples' digits are always even and add up to a multiple of three.

Seems reasonable to me. I'd say 7 is harder than 8. I think 6 is easy because it's 2 x 3 so you're already kind of familiar with the products.

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