Mental Math Shortcuts

kalid · May 15, 2014, 6:30pm

Here’s a collection of time-saving math shortcuts, great for back-of-the-envelope estimates.

60 mph = 1 mile per minute

This is a companion discussion topic for the original entry at http://betterexplained.com/articles/mental-math-shortcuts/

ajay · January 23, 2007, 8:37am

nice blog…

Anonymous_User · January 24, 2007, 5:28am

[…] Mental Math Shortcuts | BetterExplained Heres a collection of time-saving math shortcuts, great for back-of-the-envelope estimates. (tags: math) […]

bocasfx · January 24, 2007, 9:21pm

Two words: Metric System.

kalid · January 24, 2007, 9:54pm

I like the metric system as much as the next guy, but 1 foot per nanosecond just works out so nicely

peterhosey · January 25, 2007, 10:02am

Speed of light in metric: 3 decimeters per nanosecond (source). For those who don’t know, a decimeter is 1/10 of a meter; thus, another way of saying it would be 0.3 meters per nanosecond.

Anonymous_User · February 13, 2007, 12:27am

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Anonymous_User · February 13, 2007, 11:32pm

[…] Let’s say you want to add the numbers from 1 to 1000: suppose you get 1 additional visitor to your site each day - how many total visitors will you have after 1000 days? Since thousand squared = 1 million, we get […]

Anonymous_User · February 21, 2007, 1:19pm

[…] A single byte has 8 bits, or 2^8 (256) possible values. 4 bytes gives us 2^32 bits, or roughly 4 billion values. So, we could store the number 4 billion in only 4 bytes. […]

janice · March 9, 2007, 11:04am

whew! nice math shortcuts!

Anonymous_User · May 21, 2007, 4:31am

[…] The Rule of 72 is a mental math shortcut to estimate the time needed to double your money. We’re going to derive it (yay!) and even better, we’re going to understand it intuitively. […]

stevenmarzuola · July 30, 2007, 5:13am

This line is not correct:
10,000 = hundred hundred

gladysandjean · July 30, 2007, 7:16am

how to multiply in shortcuts

kalid · July 30, 2007, 3:08pm

Hi Steven, feel free try it out: 100 * 100 = 10,000 (hundred hundred is similar to “two hundred (200)”, “fifteen hundred (1500)”, “forty-seven hundred (4700)” or “hundred hundred (10,000)”. We don’t often say “hundred hundred” though :).

Gladys/Jean, I may do a follow up with mental multiplication tricks.

neilramroop · August 2, 2007, 2:32am

This line is not correct:
10,000 = hundred hundred

yes it is

Anonymous_User · August 7, 2007, 5:03pm

Electrical signals in semiconductors do not travel at the speed of light. Your 1GHz clock distance
calculation is wrong.

kalid · August 7, 2007, 7:18pm

Hi John, thanks for the info. Yes, it seems that the speed of electricity varies depending on the conductor.

http://howthingswork.virginia.edu/page1.php?QNum=1267

If there’s a factor of .66c, then it would be 8 inches per nanosecond (rather than 1 foot). I’ll update the article.

Anonymous_User · August 7, 2007, 8:27pm

two more words: METRIC SYSTEM

kalid · August 7, 2007, 10:58pm

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 :).

Anonymous_User · August 9, 2007, 4:41am

It’s misleading to say that a 16 bit number somehow equates to 64kb. A 16 bit number can REPRESENT any of 64 thousand different integers. But it is only made up of TWO bytes (8 bit bytes).