Magic

[Le Rosbif]

sasguru in Germany eating a rutabaga

[dang65]

There's one here that does not only 7 but, 6,7,8,9,10 on the fingers too, for multiplying integers >=6 and <=10.

That's clever! Trouble is, it requires a bit of mental agility to do it with any speed (I'm up to 3 minutes per calculation now, after a bit of practise).

Alternatively, for multiplying by 7, double the number and add half to the tens figure. And add 5 if the original digit was odd.
so 8 * 7 = 16 + 40 = 56, 7 * 7 = 14 + 30 + 5, 123 * 7 = feck it, you do it.

...and that one's completely done my head in.

[NotAllThere]

There's one here that does not only 7 but, 6,7,8,9,10 on the fingers too, for multiplying integers >=6 and <=10.

Or you could have invested some time when you were 7 learning your times tables.

Alternatively, for multiplying by 7, double the number and add half to the tens figure. And add 5 if the original digit was odd.
so 8 * 7 = 16 + 40 = 56, 7 * 7 = 14 + 30 + 5, 123 * 7 = feck it, you do it.

...and that one's completely done my head in.

Adding half the number to tens figure is the same as multiplying by five. As you're doing integer arithmetic, if the number is odd, then you need to add another five after you've done this. Then, you add on twice the original number.

I.e., you're just doing 2x + 10x/2 = 2x + 5x = 7x

[xoggoth]

The magazine does explain it, as some bright sorts have pointed out. Prefer magic myself.

It also says that anyone who got

Aard-Vark in Bolivia eating Kiwi Fruit

is an unstable psychotic and should be locked up before he/she kills somebody.

[dang65]

Or you could have invested some time when you were 7 learning your times tables.

Fortunately, I did, which is a great relief seeing as this is the alternative.

[TimberWolf]

Fortunately, I did, which is a great relief seeing as this is the alternative.

Amusing imagining you break your 3 minute record for multiplying 6 by 7

The one for the days in the month on your knuckles is useful. Much better than that rhyme that I can never remember.