• ColeSloth@discuss.tchncs.de
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      5 days ago

      it’s very often not useful. Like, what’s 6% of 80? See. Still hard.

      Also, with the 6% of 50 example it’s easy without using this trick.

      What’s 6% of 100? 6, right? So cut 100 in half to get to 50, and cut the 6 in half to get the 3. Cut the 50 in half again and cut the 3 in half again, and you’ll also know that 6% of 25 is 1.5.

      Which also lets you know that 6% of 75 is 4.5. the 3 from the 50, plus the 1.5 from the 25. or just know to split the difference between the 3 at 50 and the 6 at 100.

      A lot of math is just tricks for a thinking process.

      • starik@lemmy.today
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        4 days ago

        It’s always as easy as multiplying the two numbers. Just convert the percentage to a decimal.

        0.06x80 is just as easy as 0.8x6. It’s just a matter of knowing 6x8 is 48, which quickly gets you to 4.8 here.

      • Atomic@sh.itjust.works
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        5 days ago

        6% of 80. 1% of 80 is 0.8 0.86 is (86)/10 Which is 48/10 = 4.8

        80% of 6. I’d probably think of it as (4/5)*6 = 24/5 Which is 20/5 + 4/5 = 4 + 4/5 = 4.8

        Both are easy if you know how to visualise it. The benefit of knowing you can “reverse” it is that you can choose which method is easier for you to visualise. Which is incredibly useful for a lot of people

      • ddplf@szmer.info
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        5 days ago

        Good luck explaining your supreme methodology to anyone that isn’t a geek.

        • ColeSloth@discuss.tchncs.de
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          5 days ago

          It’s stupid simple… If you know 6% of one hundred is 6, then you should know 6% of 50 is 3. You just cut each number in half.

  • stickyprimer@lemmy.world
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    5 days ago

    Fuck. I had to calculate 4% of 7 a few minutes ago and didn’t know this. I say fuck because it wouldn’t have helped me anyway.

      • stickyprimer@lemmy.world
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        4 days ago

        My brain was more like “Okay it’s just seven point-fours. So what’s seven fours? 28. What’s seven point-fours? 2.8. That can’t be right. Fuck. Get a calculator.

        • qarbone@lemmy.world
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          4 days ago

          I go for finding percentage factors.

          4%? That’s just 1% * 4. 1% of 7 = 0.07 * 4 = 0.28

          14%? That’s just 10% + 1% * 4 = 0.7 + 0.07 * 4 = 0.98

          You tell me it’s decimal, then I’m gonna Decimal.

  • starik@lemmy.today
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    4 days ago

    Just always convert the percentage to a decimal, and it’s easy either way.

    0.06x50 is as easy as 0.5x6. They’re both just 5x6.

    All these comments like “What about 47% of 3.11?” Well, yeah, of course it’s harder to multiply two- and three-digit numbers than it is to multiply single-digit numbers.

  • SnarkoPolo@lemmy.world
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    5 days ago

    True mathematics derives from established mathematical theory, not some farcical aquatic armored vehicle!

  • melsaskca@lemmy.ca
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    5 days ago

    It was recently discovered that all AI everywhere was connected to the Panzer all along. /s

  • taiyang@lemmy.world
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    6 days ago

    I was waiting in a car in front of a math tutoring building and was doing the math programs advertised on their window (as one does, of course) and the 7% of 250 is indeed probably easiest as 250% of 7, aka 2.5x7, which is 17.5.

    I did it a few other ways, though. 7% of 1000 is 70, then divide by 4 for 17.5. Well, more like, divide by 2 for 35 and again for 17.5, but yeah… mental math is fun.

  • kryptonianCodeMonkey@lemmy.world
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    6 days ago

    Another counter-intuitive percentage fact: A 10 percent discount and a 10 percent fee can be calculated in either order but they don’t cancel out.

    Example, 90% of (110% of 10) = 110% of (90% of 10). But neither of those is evaluates to 10. They evaluate to 9.9.

    For any percentage X, and and base value C,(100 - X)% x ((100 + X)% x C) = (100 + X)% x ((100 - X)% x C) = (100 - (X^2/100))% x C

  • hperrin@lemmy.ca
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    6 days ago

    6% of 50

    6/100 * 50/1

    (6 * 50) / (100 * 1)

    (50 * 6) / (100 * 1)

    50/100 * 6/1

    50% of 6

    Works for multiplying any fraction, not just percents.

    2/3 of 5 is the same as 5/3 of 2.

    2/5 of 7/8 is the same as 7/5 of 2/8.

  • TrackinDaKraken@lemmy.world
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    6 days ago

    Let’s try this on a sale item. Hmm, let’s see, the sign says 25% off $6.99. So that’s the same as, let’s say, 7% off $25. Oh, yeah, that is much easier. /s

      • LordKitsuna@lemmy.world
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        5 days ago

        I mean if you remove the word off it works. 25% off $100 is $25 100% of 25 is 25 it’s still reversible you just let the word off confuse you

        • derfunkatron@lemmy.world
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          5 days ago

          You could also make sure to carry “off” to both sides when you swap:

          25% off of $100 is 100% of $25 off

    • starik@lemmy.today
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      4 days ago

      It’s always as easy as multiplying the two numbers. Obviously some pairs are easier than others. 7x25=175. Not too hard. $1.75 off.

    • Jankatarch@lemmy.world
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      5 days ago

      They said the water could reach up to 50% of its 1.7 meter height before causing problems, panzer tried calculating 1.7% of 50 meters from head, this is the result.

  • Botzo@lemmy.world
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    6 days ago

    Great for calculating tips!

    Let’s see: the bill was $138.72, so I just need 138.72% of 20. And we can subtract 100% and just add the 20 back at the end, then convert the percentage to a decimal so (.3872 * 20) + 20 = 7.744 + 20 or $27.74.

    Easy!

    • [deleted]@piefed.world
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      6 days ago

      I prefer taking 10% and doubling it for a 20% tip.

      Take the total, moved the decimal one digit to the left, double it and round to the nearest dollar. I round up at the beginning and end as well which is either close enough or a dollar high if the change is small enough.

      $138.72 > 139 > 13.9 > 27.8 > $28

      Close enough!

      • valar@lemmy.ca
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        6 days ago

        I either do this, or mentally divide by 5 (how many 5s make this number?), only caring about the closest whole number - I prefer to tip in whole dollar amounts to make the math easy.

        Eg. $39.53 is about $40, 40 is 8 fives, so tip $8

        If the service wasn’t great I round down instead of up.

        • [deleted]@piefed.world
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          6 days ago

          I multiply the tip by 5 to double check my math which is how I know it comes out as a dolllar higher sometimes.

          28x5=140

    • Catoblepas@lemmy.blahaj.zone
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      6 days ago

      I always just do 10% (easy because you move the decimal one place) and then either double that or add half of it to itself for a 15-20% tip.

      • Botzo@lemmy.world
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        6 days ago

        For sure. Reversing the percentages here makes this far more difficult. I just felt we needed to shittify this post a little.

      • Waterpumpee@lemmus.org
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        6 days ago

        i just do 0. I feel like giving 20% more than agreed upon as default is crazy. I can not imagine employers giving 20% more wages just because i was doing my job? Why should i burn money?

        • Catoblepas@lemmy.blahaj.zone
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          6 days ago

          I tip because I’ve been reliant on tips before, and I know that expecting someone who is working for tips to single-handedly solve tipping culture is unreasonable. I don’t believe in solutions that punish the poorest person in the equation, especially for a luxury like dining out.

          You can already eat at places that pay a real wage to servers if you don’t want to tip.