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

  • zephyr@lemmy.world
    58·
    2 years ago

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

    • stallmerEnglish
      2·
      2 years 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”.

  • Flyingtiger188@kbin.social
    32·
    2 years 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.

    • seeCseas@lemmy.worldOP
      1·
      2 years ago

      Here’s the technical mathematical explanation:

      Yuh-doy!

      (jk, thanks for actually posting proof)

  • MisterFrog@lemmy.world
    6·
    2 years 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

  • sourworm@kbin.social
    4·
    2 years 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.

  • glibg10b@lemmy.ml
    1·
    2 years ago

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

    Slightly better explanation: 7 × 1/100 × 50 will always equal 50 × 1/100 × 7 (the percentage sign (%) stands for /100)