I don’t know if smartphones cause brain cancer or not, but they definitely cause head trauma. In fact, the general population’s inability to do simple head math is one of the greatest collateral damages of the omnipresent calculator era.
Head math is still useful. I mean, what if you get stranded on a deserted island?
But srsly. Head math lets conversations involving simple arithmetic decisions continue fluidly. It allows you to stay hands-free while looking through the sales rack. And candidly, it just impresses people—in a non-arrogant way, which is the best way, right?
Here are a few you should know.
To multiply by 4, double the number twice.
Example: 17 x 4. Think 34, then 68.
To multiply by 5, halve the number, then multiply by 10.
Example: 72 x 5. Think 36, then 360.
To calculate a 15% tip, halve the number, then add it to the number, then move the decimal one spot to the left.
Example: 23 x 15%. Think 11.5, then 23 + 11.5 = 34.5, then 3.45.
To calculate a 20% tip, double the number, then move the decimal one spot left.
Example: 41.25 x 20%. Think 82.50, then 8.25.
Note: this is just another reason to tip well. The math is easier.
Instead of subtracting, add “up” to the higher number. It’s way easier mentally.
Example: 365 – 280. Think 20 gets me from 280 to 300, then 65 gets me to 365. Add the pieces: 20 + 65 = 85.
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http://en.wikipedia.org/wiki/Mental_calculation
If your friends are impressed by your ability to calculate 20% in your head, you have some very innumerate friends. I’m with you on tipping well, though. And I love the fact that you expect to do math on a desert island; so many of us would just swim and read Shakespeare.
Just love everything you publish. It, along with Don Mcallister, David Sparks, and a few other guys always goes to my ever loving Evernote. This post is so cool and could only come from an actuary [by the way really enjoyed your TextExpander snippet about adding commas].
The post took me back to high school which was in the 50′s. I used to get a lot of detentions. To get out of detention before the required lock-down time, one was given a number like say 1492 and an answer sheet. One had to produce the square root of that number and place it next to say 1492. One then had to find the square root of every single number from 1492 to 1592 on the answer sheet to get out of jail early.
All I had back then was a slide rule and all this frail mind remembers is I created a formula whereby once I had the first number (square root of 1492 in my example), I could get the next numbers instantly without going through any square root calculation. I could with my formula escape detention in less than 15 minutes.
I went to a very strict Catholic School run by Brothers of the Holly Cross. The Brother who ran detention was my high school math teacher (Algebra, Geometry, etc.). Very pissed after seeing what I was doing, he told me I had to do something other than the square root test. I protested and went to the principle who was a great guy. He ruled that if I had detention, I must be given the square root test to get out of jail, telling my math teacher he should be proud of what he produced in terms of me.
My grey matter has faded with the years and I cannot recall my out-of-jail formula. I know that you or somebody out there will give me the answer.
This post brought back so many memories. Keep up the good work. Sorry for my length.
Umm, all of these are actually HARDER than doing the actual math. Interesting ways of getting there, though.
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