... because multiplication is commutative.
this post was submitted on 09 Nov 2023
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69% of 420 is equal to 420% of 69.
So, is 289.8 the magic, double "nice" number?
Nice^2
Bro that's actually amazing
33% of 7 = 7% of 33
Bitch, this only works for certain easy maths.
29% of 29 is 29% of 29
Fuck, this trick sucks.
Which equals 29^2 x .01
=> ((28 x 30) +1) X .01
=> (840 + 1) x .01
=> 8.41
28 x 30 is an easy mental calculation, as is adding 1, as is moving the decimal place over 2 places. I am teaching this to 4th graders, in two weeks. % to decimal is next week. They can square 2-digit numbers in their heads, already.
((30 x 30)-30-29) x .01, easy
33% is the easy one.... 1/3 of 6 is 2, 1/3 of 1 is 0.33... 2.33..
1% of 33 is .33
.33 × 7 =
.3 x 7 = 2.1
.03 x 7 = .21
21 + 2.1 = 2.31
So
7% of 33 = 2.31
You could also do
33% of 7
10% of 7 is .7
1% of 7 is .07
So 11% is .77
.77 × 3 =
.7 × 3 = 2.1
.07 × 3 = .21
2.1 + .21 = 2.31
That's pretty damned intensive for a trick that's supposed to make the maths easier.
Who said math will be ever easy?
....I mean, yeah? If the number is 50 or 10 that works out great. But let's try that with 7% of 13. Now it's 13% of 7. Just like you said, "much easier to calculate."
Okay, choosing prime numbers was intentionally mean on my part. But 3% of 9 becomes 9% of 3. 4% of 2 becomes 2% of 4. Can anyone honestly look me in the eye and tell me that this tip has helped them out in any meaningful way?
x% × y = x × y ÷ 100 This helps you out much more.
Its easier:
10% of a 100: 100 * 0,1 = 10
2,5% of 33: 33 * 0,025 = 30 * 0,025 + 3 * 0,025 = 0,75 + 0,075 = 0,825
Thats how I calculate it mostly in my brain. But being smart I usually just type everything (like 9*9) in the calculator xD
Well, single digit percentages are easy. If it helps, move your decimal to the right so 9% becomes 90%. You can probably calculate 90% of 3 because you can do 10% and subtract it and get 2.7. Now move your decimal back to the left and you get 9% of 3 which is 0.27. You can do the same with higher percentages once you learn to break them in to 10% pieces.
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You're really asking whether commutativity of multiplication has ever helped anyone? Because that's what this is.
And yes it has helped me eg. estimate things or whatever along the years – but of course it's not going to be some sort of magical mathematics trick where just by reversing the numbers it'll always make things easier to calculate in your head
No, I think we all learned that multiplication is commutative in late elementary school, and obviously that's an important thing to know.
But I think the original post tried to make it out to be some magical mathematical trick, and I really don't understand that. Maybe I misunderstood the post.
Edit: wow, "commutative" is a really hard word to spell.
Yeah I think this is more about how you interpreted it, because it doesn't look like others took it as being an absolute magic trick rule and neither did I.
The Panzer of the Lake didn't use the word "commutativity" (fuck that really is hard to spell), but it gave out some wisdom that applied that rule by saying that "percentages are reversible": if the reverse of a percentage would be easier to calculate, you can use it and get the same answer. If it's not easier, well, then you're screwed 😁 Oooooor depending on the situation you can use the a × b% = a × b / 100 commutativity trick:
7 × 8% = 0.56
7 × 8 / 100 = 0.56
Oh, that is actually much more helpful. So, if you know your times tables, you can do percentages. You just have to use communitavatization.
Jokes aside, I really appreciate it. That made it much more easier to understand. Thank you.
You're welcome! Glad I could give you an a-ha moment.
Having an intuitive feel for tricks like using the definition of operators or symbols to make your life (well, calculations at least) easier usually means that you either have to just be "naturally" talented at math which is really rare, or you've just had to grind grind grind math at eg. university, work or whatever. So unless someone was taught that above trick and they actually remember it from school, they might never come to think that "hmm % just means / 100 so that can make this easier to figure out in my head".
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I'd find 9% of 3 easier for sure. Sometimes it's easier sometimes it's not. Just use it when it's easier.
Wait are you questioning the wisdom of panzer of the lake?!
This tip helped me 21%
So you might even say, "21% of you helped the tip"?
21% of 100%?
No. 100% of 21%
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I read that now not for the first Time
I still dont believe it
You can work it out yourself. 6% is the same as 6/100, so 6% of 50 is (6/100)*50. Then do some algebra and see if you can jiggle it to say (50/100)*6. Then replace 6 and 50 with Greek letters so it looks more convincing.
Nah, I prefer to treat my math problems like climate change. With denial.
jiggle it
aljigglebra
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You could also do 6*50 and then divide by 100.
Why would I do that
In the meme example you wouldn't, but if you were trying to figure out Something like 2% of 5, it's easier to do (2 * 5)/100=0.1 then to do .02 * 5 or .05 * 2.
My comment was mainly pointing out that you can multiply those numbers in whatever order you want.
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Thanks Panzer
Woah!
Nice username
Right back at you! :)
Also, a surprising number of people don't know that precentages are more often than not represented as decimals between 0 and 1 as opposed to actually a number out of 100 when used in calculations (because the concept of a percent doesn't really exist in math, it's just a context specific way of formatting a decimal). A lot of people just enter 69 when calculating a formula that operates on a precentage instead of 0.69 which obviously makes the formula useless, or if a formula is supposed to output a precentage, they assume that it output 0.69 percent instead of 69 percent.
Same energy as multiplication with 9, just take 1 from the right side and put it on the left
(09, 18, 27, 36, 45)
Instructions unclear...
09, 108, 1107, 11106, 111105
Keep doing this until you get a binary number and then your pc can do the rest for you
Found the JavaScript developer
You can multiply by 9 on your fingers.
Hold hands out, fingers splayed, thumbs towards each other. 🙌
9*2 put your left ring finger down (the 2nd finger). Left of that is tens place, right of that is the ones place.
So 1 on the left, 8 on the right of the downed finger.
9*5 put the left thumb down (5th finger.) 4, 5. 45.
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I thought with "reversible" they meant that a number falling 50% and then rising 50% afterwards, it is the same number, which is not true.
10 * 50% = 5
5 * 150% = 7.5
50% of 10 is 5
10% of 50 is 5
Not sure if you're saying you got it.
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