[Finance] Maths Tips and Tricks

[East Staffs Gull]

Could always multiply 12345679 by 9.

 

[clapham_gull]

I'm a huge fan of "spell checking" algorithms.

Ever had to match up two lists of things which contain "matches" but humans have spelt / expressed them in a different way ?

Jaccard Coefficient is your friend. I also like the vector space model and voted remain,

 

[KingKev]

The sum of the numbers 1 to n is (n x (n+1))/2
Eg you want to add up 1+2+3+....+10 the answer is
10x11 = 110 and divide by 2 = 55

And obviously can use the same formula for any run of numbers y to n by expanding to ((n x (n+1)) - (y x (y+1)) / 2.
the sum of the numbers 11-20 is therefore
(20x21) = 420 less 110 [ie (10x11)] = 310 and divide by two = 155
...Or you could just know that the sum of 11-20 has to be exactly 10x10 (100) more than the sum of 1-10, but that's less fun

 

[BN9 BHA]

iPhone Calculator work for me

100% this

 

[Is it PotG?]

iPhone Calculator work for me

How does an android phone calculator compare? Slower, different answers?

 

[OzMike]

Multiplying by 11.
If you have a 2 digit number who’s sum total is less than 10 then you just add the two numbers together and put the resulting number between the original ones.
Eg. 23 x 11 = 253
63 x 11 = 693

 

[Saunders]

Could always multiply 12345679 by 9.

That leads me into my one that multiples of 9 always add up to 9 except for some exceptions like 99 where they add to a multiple of 9. I also always multiply by 10 then halve it for 5x. My primary school maths teacher taught us tricks like this though I think I have forgotten more than I remember. Its why I fell in love with maths and then computing.

 

[PILTDOWN MAN]

I was taught that only work for 3!

9 is a magic number the multiplication singular numbers add up to 9’s

1x9=9
2x9=18. 1+8=9
3x9=27. 2+7=9
4x9=36. 3+6=9
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25x9=225. 2+2+5=9

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[timbha]

9 is a magic number the multiplication singular numbers add up to 9’s

1x9=9
2x9=18. 1+8=9
3x9=27. 2+7=9
4x9=36. 3+6=9
.
.
.
.
25x9=225. 2+2+5=9

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Or in this case 2 x is the magic number (see #14)

 

[father_and_son]

If all the digits of a number add up to something that is divisible by 3, then that number is divisible by 3

Only ever found that useful in the numbers round on Countdown

Likewise, if the last two digits divide by 4, the whole thing does... Handy for checking for leap years and Olympics.

 

[Nottseagull]

multiples of 9 always add up to 9 except for some exceptions like 99 where they add to a multiple of 9.

Numbers are infinite, so I think you've got that the wrong way round!

How does an android phone calculator compare? Slower, different answers?

Just the same but for a fraction of the cost.

 

[Saunders]

I had an excellent Primary School education at West Hove where we did long division etc but I wasn't taught any of those methods at any school. Is this the same for anybody else?

It might just be down to luck. I went to a poor Primary School where they took on a lot of new ideas for teaching. One of those was to not teach the times tables or long division or any of these tricks. I got lucky when a stand in deputy head arrived and saw the poor level of maths in our class and taught us all of this. He also got everyone in the school playing chess but thats another subject. It was like a revelation to me. Its not like the new methods at the time were bad but its great to have more than one way of doing it and some work better for some and vice versa.

 

[Nottseagull]

Listening to "More or Less" just now, they presented the little-known (apparently) fact that any percentage of any amount is the same the other way around, ie. 4% of 75 is the same as 75% of 4 = 3. Which I think is very cool.

Then, there's the 9x table on your fingers: with your hands out in front of you, 3x9 = drop your 3rd finger on the left, you now have 2 fingers to the left of it, and 7 on the right, so your answer is 27.

I've always like coping with dividing fractions (not a daily need, I grant you, but still): 1/3 divided by 2/5 = 1/3 x 5/2 = 5/6.

Anyone else have some trivia/shortcuts they can share?

The repeat of this program has just started on Radio 4 but I don't know when the item will be. Did you know that there are no flights between Norwich International and London?

 

[Nottseagull]

Likewise, if the last two digits divide by 4, the whole thing does... Handy for checking for leap years and Olympics.

That's a bit like saying that if the last two digits are divisible by 10, then the whole number is.... which is kind of obvious. Also, doesn't quite work for all leap years - 1900 was an Olympic year but not a leap year.

 

[symyjym]

Japanese Multiplication is interesting

https://www.youtube.com/watch?v=JVgnUMdM6ME

 

[Saunders]

That's a bit like saying that if the last two digits are divisible by 10, then the whole number is.... which is kind of obvious. Also, doesn't quite work for all leap years - 1900 was an Olympic year but not a leap year.

The true formula for leap years is divisible by 4 unless its evenly divisible by 100 it isnt...unless its evenly divisible by 400 then it is.