# Mental Math Shortcuts

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/

nice blog…

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

Two words: Metric System.

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

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.

[…] Mental Math Shortcuts […]

[…] 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 […]

[…] 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. […]

whew! nice math shortcuts!

[…] 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. […]

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

how to multiply in shortcuts

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 :).

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

yes it is

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

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.

two more words: METRIC SYSTEM

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 :).

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).