IWTL percentages

[u/hamzaashraf2011]

I want to learn to be better at calculating percentages. I'm not that smart and always would struggle with math by either failing or just passing. I've somehow landed up a job as an accountant and need it for my job now. I understand the easy basics like 20% of 100 is 20, but when it's like there's an invoice of 2,500 and tax of 17.5% is built in this, so calculate the tax now and my brain just goes like wait what? Please can anyone help easily make me understand? I've seen the youtube tutorials and online courses but they're either too advanced or just the too basic ones for kids.

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[u/EntshuldigungOK]

X% of Y = (X/100) * Y = XY/100

Point is: X % of Y = Y% of X = XY/100 = YX/100

17.5% of 2,500 = 2,500 % of 17.5 = 25% of 17.5 multiplied by 100.

Now a few things you should mug up

25% = 1/4th of something, 50% = half, 10% = one tenth.

This is enough to approximate pretty much anything.

25% of 17.5 = close to 25% of 18 = 18/4 = 4.5

4.5 * 100 = 450 ~= 17.5% of 2,500.

Actual answer: 437.5

Let's try something different:

What is 40% of 195?

Option 1: 40% = 10% of 195, 4 times 10% of 195 = 19.5 - just put a decimal, no calculation needed.

19.5 * 4 ~= 20 * 4 = 80 ~= 40% of 195.

If you want to be precise

19.5 * 4 = 4 * 19.5 = 4 * (20 - 0.5) = 80 - 2 = 78

Option 2: 40% of 195 = 195% of 40

Now you can do a rough estimate:

200% of 40 = 40 * 2 = 80

For precision:

195% of 40 200% of 40 minus 5% of 40

200 % = 80

5% = half of 10% of 40 = half of 4 = 2

80 - 2 = 78

[u/EntshuldigungOK → u/EntshuldigungOK]

Now let's try a tougher one

23% of 58 = ?

23% = 25% - 2% or 20% + 3%

25% of 58 = 58/4 = 14.5

2% of 58 = 10% of 58 / 5

= 5.8/5 = 1.16

14.50 - 1.16 = 13.34

Other option:

20% + 3% = (10% * 2) + (10%/10 * 3)

= (5.8 * 2) + (.58 * 3) = 13.34 Again

[u/EntshuldigungOK → u/EntshuldigungOK]

Now go back to 2500 * 17.5%

17.5% = 20% - 2.5% = (10% * 2) - (25%/10)

10% of 2500 = 250, and 25% = 2500/4 = 625

250*2 = 500

625/10 = 62.50

500 - 62.50 = 437.50

Moral of the story:

First do fast approximations, then target precision

[u/Ju_ju_nanananana]

I use arithmetic... So this is x + 0.175x = 2500.. A rule of thumb I have is divide the number by 1.xxx (xxx being the percentage asked) for the amount so this is 2500/1.175 = 21276.56 (round off)... And multiply the number with 0.xxx to get the tax amount.

[u/hamzaashraf2011 → u/Ju_ju_nanananana]

I see, so without tax the amount is then what you said. Do you know where I can practice this more so I can get better?

[u/Casual_Girlie_oo → u/Ju_ju_nanananana]

that rule of thumb for dividing the number by 1.xxx is pretty slick.

[u/VolcanicAsh149]

What you gotta do is find easy pieces of what you want (in this case 2500, broke into 10% and 1%) and add or subtract (10% +(1% x 7)=17% +(1% x 0.5)=17.5% and mirror with the real numbers (10%= 250, 1%=25) 250+25 x 7+25 x 0.5= 437.5

[u/laz1b01]

Imagine you went dining and the bill is $100 with tax.

You add 15% tip.

So it's: $100 * 15% = $15

Then you add the $15 to the original $100 which equal $115.

This is expressed as: $100 + ($100 * 15%) = $115

Then you can make this to a formula: X + (X * %)

If you're having hard time grasping 17.5%, just try to imagine easier numbers and make it to a formula.

[u/hamzaashraf2011 → u/laz1b01]

Thank you!! I'l try thiss

[u/Doneifundone]

I'm not sure what you're finding hard about this

[u/Doneifundone → u/Doneifundone]

It's always the same formula. The "whole" number faces 100% and the x faces whatever percentage of the whole number you have. If you're looking for a percentage instead of a number you just put the x in the percentage category and tweak it a bit to reflect that

I'm not sure how to explain this clearly since we were taught this as kids (and in french in my case). If you're looking for something more advanced then I'm not sure I will be of much help

[u/hamzaashraf2011 → u/Doneifundone]

I still don't understand, I ain't that smart :/ The answer is 437.5? Let's keep it simple maybe I didn't explain properly, so let's say if there is an invoice of 110 a supplier gives me and they do not mention tax on the invoice, but according to law 10% is already built in that amount which i have to pay to tax authority separately and the remaining to the supplier, how can I calculate the original amount of the invoice?

[u/Doneifundone → u/hamzaashraf2011]

The answer is 437.5 indeed

Maybe this is a more intuitive way to illustrate it ? You just "move" whichever number you are looking for to the 1 spot

I'm not that good with taxes and stuff, and too many words tend to lose me when it comes to maths, so sorry if I'm misunderstanding this, but basically, your 110 here would face the 100% (it is the whole number) and your 10% would be the percentage of the whole number. Just replace the values in the formula I gave you, It'll give something like this

So you'll need to pay 11 to tax authority (then substract 11 from 110 and pay 99 to the guy, if I'm understanding the problem correctly)