Why are 7 and 8 the hardest to memorize?
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.
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.
Days of the week?
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?
American football scores.
http://www.theguardian.com/news/datablog/2013/may/31/times-t...
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.