Dividing by 7 is also a cool trick and it’s way easier than this.
1/7th is 0.142857 recurring. All you need to remember is one-four-two-eight-five-seven. Say it over and over. It’s my favourite number.
The magic: to get 2/7, 3/7, any/7, just move the decimal point:
- 2/7: 0.285714285714…
- 3/7: 0.428571428571…
- 4/7: 0.571428571428… (I’m typing these out but ‘from memory’ but not really: only by knowing 1-4-2-8-5-7)
- 5/7 & 6/7: left as an exercise for the reader.
So then the trick is to play a bit of mental magic. Pretend like this is really hard and that every decimal point is stretching your mental capacity to its very edge.
31/7? That’s 4, with 3/7 left over. Say it out loud as you gaze in to the middle distance and do this difficult, difficult calculation in your head: “four point four two eight five seven … one four … two eight five … seven one four …” and so on.
A fun trick which is very occasionally practically useful.
Comments
Dividing by 7 is also a cool trick and it’s way easier than this.
1/7th is 0.142857 recurring. All you need to remember is one-four-two-eight-five-seven. Say it over and over. It’s my favourite number.
The magic: to get 2/7, 3/7, any/7, just move the decimal point:
- 2/7: 0.285714285714…
- 3/7: 0.428571428571…
- 4/7: 0.571428571428… (I’m typing these out but ‘from memory’ but not really: only by knowing 1-4-2-8-5-7)
- 5/7 & 6/7: left as an exercise for the reader.
So then the trick is to play a bit of mental magic. Pretend like this is really hard and that every decimal point is stretching your mental capacity to its very edge.
31/7? That’s 4, with 3/7 left over. Say it out loud as you gaze in to the middle distance and do this difficult, difficult calculation in your head: “four point four two eight five seven … one four … two eight five … seven one four …” and so on.
A fun trick which is very occasionally practically useful.
that's really cool. but how does it help determine the day of week?
I don’t follow. How does this calculate the day of week?
Not one part of this makes any sense to me.