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[Finance] Maths Tips and Tricks



Brightonfan1983

Tiny member
Jul 5, 2003
4,803
UK
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?
 


pasty

A different kind of pasty
Jul 5, 2003
30,180
West, West, West Sussex
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 :lolol:
 


JimothyJames95

New member
Jan 2, 2019
98
Eastbourne
I know for Trigonometry, when figuring out angles and lengths if you use SOH CAH TOA for the various sides of the triangle in relation to the angle. So if you use S = Sine and put the Opposite (O) over the Hypotenuse (H). Works for the other ones as well. Helped me when I was at school for my GCSE’s and with my job doing drawings for some certain designs for unknown lengths which were unable to be measured.


Sent from my iPhone using Tapatalk
 










Brightonfan1983

Tiny member
Jul 5, 2003
4,803
UK
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 :lolol:

Oh that's good. On a recent Fish podcast, they were discussing prime numbers. They are searched for now with the formula (2x2x2....)-1. Each result can also be written as 1111.... in binary. Mental.

*Not that you were talking about prime numbers, it just reminded me...
 








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?
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?
 
Last edited:


Brightonfan1983

Tiny member
Jul 5, 2003
4,803
UK
Very basic, but if you want to multiply a number by 5, it's easier to halve it then multiply it by 10.

eg. 1038 x 5

1038 / 2 = 519

519 x 10 = 5190

So basic that I bet not that many people know it as an easy shortcut.

I work as a supply teacher occasionally and I can guarantee you, the easiest ways in maths are never taught. I shall use that deffo :thumbsup:
 






timbha

Well-known member
Jul 5, 2003
9,834
Sussex
If the digits of a number add up to a number divisible by 9, then the original number is divisible by 9
Eg
27 = 2+7 = 9
Or
999 =9+9+9=27=2+7=9
 






studio150

Well-known member
Jul 30, 2011
29,555
On the Border
So basic that I bet not that many people know it as an easy shortcut.

I work as a supply teacher occasionally and I can guarantee you, the easiest ways in maths are never taught. I shall use that deffo :thumbsup:

Really depends on how good you are at mental arithmetic, but to me just seems like an unnecessary extra step when multiplying by 5 which is straightforward.

No doubt most today would just reach for a calculator
 




Really depends on how good you are at mental arithmetic, but to me just seems like an unnecessary extra step when multiplying by 5 which is straightforward.
Unless you are Rainman, you don't know your 1038 times table so I put it to you that you are only capable of working that out in your head by splitting it, probably as (5x1000)+(5x38).
 






Creaky

Well-known member
Mar 26, 2013
3,835
Hookwood - Nr Horley
The only ‘tip’ that I have used in practice is the 3 4 5 triangle.

A triangle with sides of 3 units, 4 units and 5 units has a right angle between the 3 and 4 sides.

Useful when wanting to layout a right angle for landscaping etc.
 





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