Nice! I’m going to link this on my Facebook page!
It is so useful and so helpful for me. Thanks a lot. If you can, please send me more helpful tricks to my e-mail ID sir.
I believe this line has a typo:
Have 32-bit color? That’s 2 + 30 bits = 2^2 * 2^30 = 2^2 billion = 4 billion (4gb exactly).
2^2 billion should be 2 * 4 billion. It should read:
Have 32-bit color? That’s 2 + 30 bits = 2^2 * 2^30 = 2 * 2 billion = 4 billion (4gb exactly).
Disregard my last comment. I believe you were expressing (2^2) billion, not 2^2,000,000,000.
@MD: Glad you enjoyed it – easiest way to stay updated is to sign up for the email updates.
kya explanation h lekin or acha hona chaiya
[…] Rule of 72 is a great mental math shortcut to estimate the effect of any growth rate, from quick financial calculations to population […]
“Interest rate” can be generalised to “percentage per year”, and applied to such things as population growth, crime statistics, etc. So: Years To Double = 72 / %p.a.
(or, more accurately, Years To Double = 70 / %p.a.).
Great blog, just stumbled upon while looking up Rails tuts. Have bookmarked. Thanks.
A very handy collection. I’d like to feature and quote you in my site if you agree. Thanks! 
Hey, do you have similar shortcuts for the metric system? Thanks!
Kalid, Hi:
The subtitle on the selection button for ‘Mental Math Shortcuts’ is ‘A few memorable converions’. Did you mean ‘conversions’?
[…] Rule of 72 is a great mental math shortcut to estimate the effect of any growth rate, from quick financial calculations to population […]
Kalid
August 7, 2007 at 3:58 pm
I love the metric system too, but 1 foot per nanosecond just works out well, don’t you think?
30 centimeters per second doesn’t have quite the same ring to it :).
30CM/NSEC SOUNDS MORE ACCURATE !!
Love the articles, Kalid!
Just a quick suggestion.
This line:
“10,000 = hundred hundred million = thousand thousand billion = thousand million trillion = million million”
While mathematically correct, it doesn’t really clearly illustrate what you want to say. Doing a quick once-over, it actually looks like you’re trying to say that “10,000 = hundred hundred million”. Might I suggest adding some sort of separator? For instance:
“10,000 = hundred hundred | Million = thousand thousand | Billion = thousand million | Trillion = million million”
To my eyes at least, this makes more immediate sense. Of course, I realise that once one reads the title slowly it makes perfect sense - but any hurdles removed might be a good thing, no?
Anyway - keep up the magnificent work!
Thanks Nicolay, that was a formatting error, just fixed!
Raj says:
How to calculate x^y manually, is there any short cuts?
For x^2 (perfect squares)
I usually use two rounding tricks to help:
- If x ends in a 5, then
(x+5)(x-5) then add 25
115115 = 120 * 110 = 13200 plus 25 = 13225
- If x is closer to 10 then round up or down to the closest 10
9898 = 100100 = approximately 10000
It’s fairly easy to compute the specific value from there
10000 - (2100) = 9800 - subtract 2 * rounded x
9800 - (298) = 9604 - then subtract 2 * original x
or
x=original number, y= difference
9898 = 100100 = approximately 10000
10000 - (2 * difference from x) * (rounded version of x) then add (difference from x^2)
10000 - (22) * (100) + (22) = 10000 - (4*100) + 4 = 10000 - 400 +4
=9604
[If you rounded down then you would add (2 * difference from x) * (rounded version of x) instead of subtracting…]
Please ignore “x=original number, y= difference” above.
I was going to write out a simpler formula:
where x=98, y=2
9898 = 100100 = approximately 10000
10000 – (2 * y) * (rounded version of x) then add (y^2)
10000 – (22) * (100) + (2^2) = 10000 – (4100) + 4 = 10000 – 400 +4
=9604