• thisfro@slrpnk.net
      1241·
      3 years ago

      They probably did, just not explicitly:

      You could write (6*1/100)*50 = 6*(50*1/100)

      It only uses the commutative property of multiplication and the fact that % is another way of writing 1/100.

      Maybe also worth remembering that “x% of y” is just x/100*y

      • Capt. Wolf
        18·
        3 years ago

        I totally remember being taught this. It’s just way easier to break down percentages in terms of the nearest 1% or 10% times the number in the percent times the number you’re taking the percentage of. You don’t have to do the math for the 1 or 10 percentage as long as you remember that a 10% means move the decimal left once and 1% means move the decimal left twice. The rest is just basic multiplication.

        40% of 59 = 10% of 59 times 4.

        So…

        4x59=236

        or

        (4x50=200) + (4x9=36)= 236

        10% means move the decimal left once,

        Therefore 40% of 59 is 23.6

        With that you can easily do more complex percentages mentally like…

        62% of 35 = 10% of 35 times 6 plus 1% of 35 times 2.

        35x6=180+30=210 at 10% so 21

        plus

        35x2=60+10=70 at 1% so 0.7

        Therefore 62% of 35 = 21.7

        • _cnt0@sh.itjust.works
          21·
          3 years ago

          While I get your sentiment, I’m always baffled how people fail to just memorize some basic formulas/equations and then just to plug and play:

          1÷kⁿ = k⁻ⁿ

          % = 1÷100 = 10⁻²

          k×10ⁿ equals k with its floating point shifted by n to the right for positive n, or to the left for negative n

          That’s really all one needs to know for the “problem” at hand. For your concrete example of “40% of 59” that would just be

          59×40×10⁻²

          Just solve that whatever way is easiest. I don’t get why people get panic-stricken when they see the % sign.

          • jasondj@ttrpg.network
            1·
            3 years ago

            They taught you all the parts. Where they (and I’d agree most math education) failed was to connect the dots.

            They taught you about these properties.

            They taught you that division is just fractions and vice versa.

            They taught you that x/1=x.

            They taught you multiplying fractions as (numerator_a • numerator_b) / (denominator_a • denominator_b).

            They taught you percentages are just “per centum”, or per hundred, or basically just a fraction “over 100”.

            But these tricks, much like many other mental math shortcuts that are useful for everyday life, got glossed over or missed entirely.

    • Sludgeyy
      74·
      3 years ago

      Easy way to do %

      Say you want 6% of 45

      Seems hard right?

      1% of 45 is .45

      .45 × 6 =

      .4 × 6 = 2.4

      .05 × 6 = .3

      2.4 + .3 = 2.7

      So 6% of 45 is 2.7

      Extra:

      Say you want an item that is 40 dollars and it is 20% off.

      10% is 4 dollars.

      20% is 8 dollars.

      So item would be 32 dollars.

      • plz1English
        11·
        3 years ago

        that’s a shitload of lines of math to write out/work in your head. I learned percentages of x as: 6 * 45 / 100 = x (2.7) If you picture both as fractions, you multiply the opposite and then divide by the other number to get the missing one (x). Hopefully Lemmy renders this well…

        6 x
        100 45

        The way I learned it was multiply diagonally and then divide by whatever is opposite diagonally to x.

      • jasondj@ttrpg.network
        3·
        3 years ago

        Honestly I recently switched to vyvanse and I don’t actually smoke to get high (at least not until the kids are in bed). I just microdose a bit throughout the day and it balances out the vyvanse. Like, the stimulants alone are just a little bit too much for me. The combo, though, I can dial in just right.

        But weed alone always made me fixate on arithmetics. And then stims turn that up to 11.

  • TAYRN
    4716·
    3 years ago

    …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?

      • plague-sapiens
        12·
        3 years ago

        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

    • Tikiporch
      22·
      3 years ago

      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.

        • Mr_Dr_Oink
          181·
          3 years ago

          This is not mental gymnastics.

          This is a coherent method and makes a lot of sense.

          Mental gymnastics is when someone has to lie to themselves to make a point that isn’t correct. Like when people argue that trump was a good president because they can list several good things he did.

          Or when people claim something is mental gymnastics when it’s actually called maths.

      • TAYRN
        4·
        3 years ago

        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.

      • jasondj@ttrpg.network
        2·
        3 years ago

        9% of 3 is easier to estimate because you know it’s “almost 10% of 3”. Or, since 10-1==9, you could think of it as (10% of 3)-(1% of 3) and get the right answer using some other shortcuts. Humans being generally pretty good at base10, this is easy to figure out in your head as (0.3 - 0.03) and get 0.27.

        Or, you could do what another commenter suggested and “3% of 9” can broken down as (3/100)•(9/1), becomes, (3•9) / (100•1), becomes 27/100, becomes 0.27. And that can be simplified as xy/100.

        Different tools for different jobs. Base10 tricks are good for stuff like figuring out, say, a 15% or 20% tip, because you can easily figure out a 10% tip just by moving the decimal one space to the left, and add half of that (for 15) or double it (for 20). Or half and half again for (almost) 18%. xy/100 is a good trick for figuring out small percentages like sales tax (unless you’re in a place like Mass where it’s 6.25 and you gotta change it now to 625y/10000. At that point I’d just estimate at 6 in my head, or if I had to solve it mentally do (6y100) + ((1y100)/4).

    • 1847953620
      512·
      3 years ago

      Yes, idiot. Choosing instances of when it’s less helpful (arguably) doesn’t negate the cases where it is deemed very helpful.

  • HiddenLayer5@lemmy.mldeleted by creatorEnglish
    111·
    3 years ago

    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.

  • cmhe
    129·
    3 years ago

    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

    • Sylvartas
      1·
      3 years ago

      Yeah the correct term would be “commutative” as someone already pointed out. Meaning the order doesn’t matter when considering multiple percentages. E.g 50% of 73% of something is the same as 73% of 50% of that same thing