247

Why YSK: It helps when calculating percentages. If you want to find out what 2% of 20 is, do 20% of 2 instead and you'll get 0.4. So, 2% of 20 also equals 0.4.

Or, 12% of 25 can be alternatively written out as 25% of 12 instead, which would be 3.

Explanation: Commutative Property. So, 0.07 times 50 will always equal 50 times 0.07

top 18 comments
sorted by: hot top controversial new old
[-] Flyingtiger188@kbin.social 26 points 1 year ago

This isn't the only property that makes mental math easy. What is 3.5% of 62? Inverting the percentages doesn't really help but if we realized 3.5% = 1%+1%+1%+0.5% we have some easy math. This gives us 0.62+0.62+0.62+0.62/2=2.17

Also remember there isn't a teacher sitting over your shoulder to check your work, most of the time you do mental math, getting close is good enough. 62 is close to 60 and 3.5% is close to 4, and 60 x 4% is easy math to get an answer that will be a little less than 2.4.

[-] zephyr@lemmy.world 20 points 1 year ago

Percentage is just a fancy form of decimal point. I just multiply them together then move decimal digit to the left twice.

[-] stallmer@lemmy.one 3 points 1 year ago

This is true for quite a few other things as well. For instance, “hertz” really just means “per second”. However, it’s easier to write/type “Hz” instead of “s^-1”.

[-] Otome-chan@kbin.social 1 points 1 year ago

damn all these good tips for doing quick percentage math @.@

[-] Guest_User@lemmy.world 12 points 1 year ago

Okay this is something everyone should know. Great tip!

[-] seeCseas@lemmy.world 2 points 1 year ago
[-] firebreathingbunny@kbin.social 8 points 1 year ago

Proof:

x/100 * y = xy/100 = y/100 * x

[-] seeCseas@lemmy.world 1 points 1 year ago

Here's the technical mathematical explanation:

Yuh-doy!

(jk, thanks for actually posting proof)

[-] KnittingTrekker@kbin.social 6 points 1 year ago

Damn... You just blew my mind O_O

[-] seeCseas@lemmy.world 2 points 1 year ago

I aim to please ;)

[-] MisterFrog@lemmy.world 5 points 1 year ago

This is actually so useful, thanks! I feel I used head maths less and less (I'm an engineer and rarely need to do a calculation without a calculator) but this will be an excellent sanity check, or a quick calc when out and about with prices or something.

Many thanks for pointing out something that ought to be obvious haha

[-] seeCseas@lemmy.world 1 points 1 year ago

It wasn't obvious until I saw this also. No judgement!

[-] Hedup@lemm.ee 4 points 1 year ago
[-] UnderpantsWeevil@lemmy.world 1 points 1 year ago

Transitive property of multiplication is crazy, dawg

[-] indite@lemmy.dbzer0.com 1 points 1 year ago
[-] sourworm@kbin.social 0 points 1 year ago

I don't see how that explanation makes sense. It's not showing the same thing.

The example in the title is saying .50 times 7 is equal to .07 times 50 which is not just changing the order like in your explanation.

[-] exscape@kbin.social 2 points 1 year ago

They're leaving out one step.
.50 * 7 = 50 * 1/100 * 7 = 50 * 7/100

this post was submitted on 18 Jun 2023
247 points (99.2% liked)

You Should Know

32980 readers
774 users here now

YSK - for all the things that can make your life easier!

The rules for posting and commenting, besides the rules defined here for lemmy.world, are as follows:

Rules (interactive)


Rule 1- All posts must begin with YSK.

All posts must begin with YSK. If you're a Mastodon user, then include YSK after @youshouldknow. This is a community to share tips and tricks that will help you improve your life.



Rule 2- Your post body text must include the reason "Why" YSK:

**In your post's text body, you must include the reason "Why" YSK: It’s helpful for readability, and informs readers about the importance of the content. **



Rule 3- Do not seek mental, medical and professional help here.

Do not seek mental, medical and professional help here. Breaking this rule will not get you or your post removed, but it will put you at risk, and possibly in danger.



Rule 4- No self promotion or upvote-farming of any kind.

That's it.



Rule 5- No baiting or sealioning or promoting an agenda.

Posts and comments which, instead of being of an innocuous nature, are specifically intended (based on reports and in the opinion of our crack moderation team) to bait users into ideological wars on charged political topics will be removed and the authors warned - or banned - depending on severity.



Rule 6- Regarding non-YSK posts.

Provided it is about the community itself, you may post non-YSK posts using the [META] tag on your post title.



Rule 7- You can't harass or disturb other members.

If you harass or discriminate against any individual member, you will be removed.

If you are a member, sympathizer or a resemblant of a movement that is known to largely hate, mock, discriminate against, and/or want to take lives of a group of people and you were provably vocal about your hate, then you will be banned on sight.

For further explanation, clarification and feedback about this rule, you may follow this link.



Rule 8- All comments should try to stay relevant to their parent content.



Rule 9- Reposts from other platforms are not allowed.

Let everyone have their own content.



Rule 10- The majority of bots aren't allowed to participate here.

Unless included in our Whitelist for Bots, your bot will not be allowed to participate in this community. To have your bot whitelisted, please contact the moderators for a short review.



Partnered Communities:

You can view our partnered communities list by following this link. To partner with our community and be included, you are free to message the moderators or comment on a pinned post.

Community Moderation

For inquiry on becoming a moderator of this community, you may comment on the pinned post of the time, or simply shoot a message to the current moderators.

Credits

Our icon(masterpiece) was made by @clen15!

founded 1 year ago
MODERATORS