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Everyday Shortcuts

Posted 07-14-2017 at 04:33 PM by rtmistler
Updated 09-07-2017 at 09:09 AM by rtmistler

A recent blog posting highlighted to me a former shortcut.

This one is supposed to be fun and not "in your face", or "I'm smarter than you!", just some stuff that comes to mind over the years. Oddly enough, sometimes I comment about these things, to people my age, and they have no idea about them. You'd figure ... but then again, I guess we all have variations in our teachings and life experiences. I guess I also had good math teachers.

These are shortcuts I've learned in life. Many are mathematical, however some are not. I'll probably remember some over time.

The Days in the Months and Weekdays Per Year

Can you tell me how many days in February? EVERYONE thought their answer and they're perfectly correct.

What about June? Takes a bit ... unless it is now June.

Short answers: "Alternating" and "Your knuckles"

Your knuckles alternate between high and low. There are however seven spots (not counting your thumb) Touch your knuckles and the valleys between them, and I mean the knuckles closest to your hand. No fair if you've lost some fingers! (Yes I have a friend who has lost fingers and he jokes that I don't know any real tricks!)

Recite the months starting from January.

January - High knuckle (31 days)
February - Valley between knuckles (28/29 days)
March - High (31 days)
April - Valley (30)
May - High (31)
June - Valley (30)
July - High (31)
Oh-ohh ... out of knuckles! Go back to the start!

August - High (31)
September - Valley (30)
October - High (31)
November - Valley (30)
December - High (31)

Yes it works. Ultimately what you need to learn/remember is that they alternate with a single exception, July to August are two months in a row with 31 days. Everything else alternates, and over time you just "know" that January has 31, or February has 28 or 29 and then the next nearby months have the alternatve.

So what about the weekdays per year? Excepting leap year, as you go from year to year. Any given day moves back forwards one weekday from year to year. Say this year your birthday was on a Tuesday. Well, next year it will be on a Wednesday. If there was a leap year, AND your birthday is after the end of February, in this case, the day does not move. EDIT: I had it backwards - the day moves forwards from year to year.

Other ones are Christmas and New Years. The spacing of the days are 7 apart. Thus, Christmas Eve and New Years Eve will always be on the same day, just one week apart. And also Christmas and New Years.

Not amazing stuff, but if you don't "think" about it, you never really lose your dependence on a calendar or a calendar app.

Math Precedence

This one actually got me started here because I wrote about it in another blog entry.

Long ago I learned in school, "Please My Dear Aunt Sue", Parentheses, Multiplication, Division, Addition, Subtraction. This is standard mathematical precedence, from left to right.


Another school thing was another Math teacher who went on about King SohCahToa (So-Kah-Toe-Ah)

Sine - Opposite over Hypotenuse (SOH)
Cosine - Adjacent over Hypotenuse (CAH)
Tangent - Opposite over Adjacent (TOA)

More Math - Each Shortcuts

Divisible by 2: even number (yeah , we all know that!)
Divisible by 3: sum of all the digits is divisible by 3
(45: 4 + 5 = 9, 9 is divisible by 3, therefore 45 is divisible by 3)
(What about 387420489? Add all the digits, they come to 45. Well, we've already verified that 45 is divisible by 3, however if the adding of the digits is still an unknown ... add those resulting digits and do it again.)
Divisible by 4: If the LAST TWO digits are divisible by 4.
(48 is divisible by 4, trust me, we know this, we can also readily figure this out with little effort even if we have to write it down)
(What about 1048, 948, 10000048? YEP! Last two. Don't believe me, use your calculator)
Divisible by 5: If the last digit is a 5 or a 0
Divisible by 6: Note that 2x3 is 6. If a number is divisible by 2 AND divisible by 3, it is then divisible by 6.
Divisible by 7: Sorry! Even my Math teacher had no quick one for this
Divisible by 8: Same thing, no quick one, however testing for divisible by 4 helps, but does not solve.
Divisible by 9: Similar to 3. Add up the digits and if the sum of the digits is divisible by 9, the whole number is divisible by 9.
10 = obvious, if it ends in 0

Resistor Color Code

I won't write it because the one I learned was not politically correct.


There are some overlaps between Black, Brown, and Blue
Also Green and Grey, you sort of had to remember, and know that Violet means Purple.

Those who know some memory variations will recall ways to remember, but as I've said the one I learned was non-PC.

Multiply by 15, or compute 15% as well as 20%

Tipping. Common may be 15%, I exceed but I probably do think about what 20% is so I don't go nutty. Meanwhile if I'm paying for good service, for a $7.25 meal tab, and I leave $10, I'm not going to break myself up about it!

Times 15: Times 10, plus half over. What's that mean? Add a zero, cut that in half and add the half to it.

How does this translate to 15%, move the decimal places over.

Becomes 450, half is 225, total is 675, tip of 15% is $6.75
450 x 2 becomes 900, 20% tip would be $9.00. Say the service is very good, maybe leave the $9.00, or maybe leave $8.00.

Salary vs. per Hour

52 weeks in a year, U.S. typical is 2 weeks vacation unless you've worked there for some time.
40 hours per week.
You work 2,000 hours per year.

Hourly: Multiply your hourly rate times 2 and add 000 to the end.

$15/hr becomes $30,000 per year. On average - people get overtime, or other stuff.

Meanwhile, say you're offered a salary, of $50,000 but you're used to being paid hourly. So what does that translate to per hour?

Cut away the thousands, and divide by 2:

50,000 becomes 50
Divide by 2, becomes $25/hour average hourly rate.

There are plenty of others

In construction I've done amateur stuff like build a shed or a deck and you learn to make an 8 foot high wall you do NOT buy 8 foot studs and hammer the headers onto either end. Yes you now have a 8' 3" some odd wall which cannot be covered with plywood properly. Further, NO two studs are the exact same length! Meanwhile the people who do this everyday, ... they just "know"

(We're not going to talk about framing a door!!!)

Think about stuff people, one thing I have learned is that those who use their brains tend to be able to be resistant to mind degradation, Alzheimer's, dementia, ... or at least medical research shows some indication that this is true. ... I'm hoping!
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  1. Old Comment
    Good tips, thanks. Just googling by "divisible by 7" presents as the first result an excerpt from some site:

    Take the last digit of the number you're testing and double it. Subtract this number from the rest of the digits in the original number. If this new number is either 0 or if it's a number that's divisible by 7, then you know that the original number is also divisible by 7
    This other site presents yet an weirder one:

    Test #1. Take the digits of the number in reverse order, from right to left, multiplying them successively by the digits 1, 3, 2, 6, 4, 5, repeating with this sequence of multipliers as long as necessary. Add the products. This sum has the same remainder mod 7 as the original number! Example: Is 1603 divisible by seven? Well, 3(1)+0(3)+6(2)+1(6)=21 is divisible by 7, so 1603 is.
    Math is just weird.
    Posted 07-19-2017 at 09:50 PM by the dsc the dsc is offline
  2. Old Comment
    Just addition in Math Precedence which I learns in my school days,

    BODMAS, which stands for Brackets, Orders or pOwers, Division, Multiplication, Addition, Subtraction.
    Posted 07-21-2017 at 12:30 AM by rockstar05 rockstar05 is offline
  3. Old Comment
    Originally Posted by the dsc View Comment
    Good tips, thanks. Just googling by "divisible by 7" presents as the first result an excerpt from some site:

    This other site presents yet an weirder one:

    Math is just weird.
    Thanks for the additional references.
    Posted 09-07-2017 at 09:06 AM by rtmistler rtmistler is offline


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