Calculating GST at 15% - Excel

Looks like 7 decimal places to me.

If I have something worth $72.11 after adding 15.23568%, what was the original amount biker? (Hint: the solution involves using the same basic skills as fractions, but complicated by using a prissy decimal instead of simple integers)

geek_cybertao, Oct 29, 7:36 pm

I want to know how this clever dick works out the GST on an inclusive amount in 1 calculation, without using fractions, and using only 15 or .15.

geek_wotz_it_2_ya, Oct 29, 8:34 pm

Hi, thanks for playing.

geek_cybertao, Oct 29, 8:38 pm

I said the GST amount, not the GST exclusive amount.

geek_wotz_it_2_ya, Oct 29, 8:39 pm

Oh.

GST = GSTinc - GSTinc / 1.15

Equivalent to GST = GSTinc *3 / 23 ...is there any way to do it in one operation, short of GST * 0.130434783?

geek_cybertao, Oct 29, 8:53 pm

Oh boo hoo. Still get the right answer.

geek_honeybean, Oct 29, 8:57 pm

No, you don't.
115 / 7.66666 = 15.000013043

geek_cybertao, Oct 29, 9:01 pm


Duh!Not the sharpest knife in the draw.

geek_radiowaves, Oct 29, 9:02 pm

excuse me - but how do you pay 15.000013043 to the IRD?
duh yourself !

geek_honeybean, Oct 29, 9:29 pm

ffs that was his point, you don't. So you don't use calculations ( your / by 7.66666 ) that are going to give you those types of answers if you don't have to.

geek_ross1970, Oct 29, 9:41 pm

You think using / 7.6666666, rounding error and all, is easier than / 23 * 3?

geek_cybertao, Oct 29, 9:42 pm

untwist your knickers!I use this on my excel spreadsheet - but have it set to 2 decimal points - therefore I get the right answer.
Why is everybody so uptight?
at the end of the day so long as you are paying the IRD the correct amount on the day - who cares how you work it out?
*leaves to do better things with my time*

geek_honeybean, Oct 29, 9:52 pm

*waves goodbye*

geek_radiowaves, Oct 29, 9:53 pm

...taking a night class in mathematics at the local intermediate school?

geek_cybertao, Oct 29, 9:56 pm

/me enrolls them.

geek_johnf_456, Oct 29, 10:02 pm

Simple. $72.11/1.1523568=$62.58.

Now do it with integers.

geek_biker_69, Oct 29, 10:06 pm

Oh and here's another one for you to demonstrate your unneccessarily complicated illogical method. A product has a 16.34% markup on the GST exclusive whiolesale price and sells for $123.65 incl 15% GST retail.

What did it cost the retailer excluding GST (which of course they don't actually pay)?

geek_biker_69, Oct 29, 10:13 pm

72.11 / 720223 * 625000 = 62.57

geek_cybertao, Oct 29, 10:24 pm

123.65 / 5817 * 817 / 23 *20 = 92.42

geek_cybertao, Oct 29, 10:31 pm

I should have said show full workings. Like the bit where you get the 720223 and 625000 from. Then you'll see how long winded it is.

I would simply use the numbers I was given - it's much faster. $123.65 / 1.15 / 1.1634 = $92.42.

geek_biker_69, Oct 29, 10:41 pm

No. $62.58.

geek_biker_69, Oct 29, 10:43 pm

You don't understand it, yet argue fractions are inferior?

geek_cybertao, Oct 29, 10:45 pm

I fully understand it. Not that I need to. Show full workings so others don't follow your stupid convoluted example.

geek_biker_69, Oct 29, 10:46 pm

Rounding error.That's why it's best not to complicate things more by using decimalisations when you don't have to.

geek_cybertao, Oct 29, 10:49 pm

It was your stupid convoluted example, I simply solved it.

geek_cybertao, Oct 29, 10:50 pm

So you're not going to show your full workings because you know it'll make you look stupid. I thought so.

geek_biker_69, Oct 29, 10:53 pm

Biker, there is nothing wrong with your logic. However, with 12.5% GST it was easier to remember to divide by 9 to get the GST amount from an inclusive figure than to muliply by .111111111111. To some people, x3/23 is easier to remember than multiply by .130434. Just depends how you think

geek_wotz_it_2_ya, Oct 29, 10:58 pm

Or more accurately, don't think.

Explain how it is easier to divide by 9 when the number has no resemblence to the GST content of 12.5%?

You know that after adding 12.5% to 100% the item is now cost 112.5%. So you just divide by that to get the GST exclusive amount and the difference is the GST content!
$123.65-(123.65/1.125)=$13.74 GST.

It works for ANY percentage by using the percentage itself to do the calculation.

geek_biker_69, Oct 29, 11:04 pm

95223 / 625000 = 0.1523568
720223 / 625000 = 1.1523568

What more do you need worked out?They are the same numbers but as fractions instead of prissy and more error prone decimals.

geek_cybertao, Oct 29, 11:05 pm

12.5% is 1/8th.When you add 1/8th to something you end up with 9/8ths.Break that into 9 and you have the value of the 12.5% you added.Multiply that by 8 and you have your original price again.

geek_cybertao, Oct 29, 11:10 pm

OK, I will explain. GST at 12.5% = 1/8.
Seeing as we are using 8 as the denominator, the exclusive amount = 8/8.
Adding 1/8 GST gives 9/8 as the inclusive figure.
The GST amount of this inclusive amount = 1 part of the 9 (the other parts being the exclusive price), so GST = inclusive *1/9.
The *1 part can be eliminated, so just /9

It was the same when GST was 10% -/11 to get GST of inclusive.

15% doesn't work as nicely -3/23 is the fraction

If you want to do 2 calculations to work out the GST, feel free. Just accept that others work it out a different way, but get the same answer.

geek_wotz_it_2_ya, Oct 29, 11:15 pm

More error prone decimals yet they give the exact same decimals only with several extra steps for your method. Percentages ARE decimals.

You're the convolution king!

geek_biker_69, Oct 29, 11:15 pm

LOL @biker.GST calculations have always been done that way at 12.5%.Some people just don't get fractions!

geek_radiowaves, Oct 29, 11:15 pm

No, they haven't "always been done that way". What is dividing by 9 if it's not effectively a fraction?

geek_biker_69, Oct 29, 11:16 pm

I know how and why it works. But it doesn't work for every percentage. The method I've outlined does.

geek_biker_69, Oct 29, 11:17 pm

Every percentage can be represented as a fraction.Every percentage IS a fraction, out of 100.

geek_cybertao, Oct 29, 11:20 pm


FFS dividing by 9 is a fraction.If you have ever done a GST return (prior to the rate change) that is what the form tells you to do.Give it up already.

geek_radiowaves, Oct 29, 11:21 pm

It says that because it works and people are stupid.

Why don't you give up pushing your imbecilic one-case method and actually learn what it is you're really doing?

geek_biker_69, Oct 29, 11:24 pm

Which is a decimal. But you claim those to be inaccurate.

What a fool.

geek_biker_69, Oct 29, 11:25 pm

Got the rest of your workings together yet to show how long and pointlessly convoluted your method is?

geek_biker_69, Oct 29, 11:26 pm

No wonder he drives the way he does.With his poor math ability to grasp physics coupled with the attitude.

geek_cybertao, Oct 29, 11:26 pm

I run rings around you in both scenarios.

geek_biker_69, Oct 29, 11:27 pm

Where's the rest of your workings? Go on, post it in its entirety and look more of a fool.

geek_biker_69, Oct 29, 11:28 pm


Lost cause.Doesn't quite comprehend that fractions are inherently more accurate than decimals.

geek_radiowaves, Oct 29, 11:29 pm

When did you last see a percentage expressed as a fraction?

geek_biker_69, Oct 29, 11:35 pm

C'mon! Where the rest of the workings, cybertoss?

geek_biker_69, Oct 29, 11:35 pm

Sorry, but fractions are no more accurate when you started with a decimal.

geek_biker_69, Oct 29, 11:36 pm

FFS if you are involved this is you .......... these few words spring to mind:P
Arrogance is a defense mechanism used by the subconscious mind in order to prevent further criticism. If someone had a terrible childhood and if he was hurt by others he may develop arrogance in order to prevent further criticism from hurting him, the trick usually works, because if someone criticized him he can simply devalue him and assume that he is worthless.

Arrogant behavior can be a result of feeling neglected. If someone felt that he is not getting all the attention he deserves, he may unconsciously become arrogant just to attract some of the lost attention.

Arrogant people are single minded, they either think that they are superior to others or inferior to them. This arrogant person who is intimidating you feels inferior to someone else because this is how his mind works, this arrogance may be nothing more than a way to cover this feelings of inferiority he experiences when dealing with someone else.

geek_dino7, Oct 29, 11:37 pm

Blah blah. Save your amateur psychobabble for someone who cares.

geek_biker_69, Oct 29, 11:40 pm

Cyberclown seems to have run away. Good night!

geek_biker_69, Oct 29, 11:40 pm