Edit - MathOverflow

You are not logged in. Your edit will be placed in a queue until it is peer reviewed.

We welcome edits that make the post easier to understand and more valuable for readers. Because community members review edits, please try to make the post substantially better than how you found it, for example, by fixing grammar or adding additional resources and hyperlinks.

Required fields*

Rev

$\begingroup$ I think something is missing from this explanation, as the method doesn't seem to work quite reliably. 11x42 comes out as 84+336=420, which is not correct $\endgroup$
– davidcl
Commented Jun 7, 2017 at 21:06
3

$\begingroup$ @davidcl: $11 \; 42$, $5 \; 84$, $2 \; 168$, $1 \; 336$, and then $42 + 84 + 336 = 462 = 11 \times 42$. $\endgroup$
– Joseph O'Rourke
Commented Jun 7, 2017 at 21:43
1

$\begingroup$ @JosephO'Rourke This method is very commonly used in markets in some Asian country ( Korea ?). think). At least before pocket calculator. A peasant way of calculating fast. $\endgroup$
– Jérôme JEAN-CHARLES
Commented Oct 17, 2023 at 18:43

Add a comment |

Correct minor typos or mistakes
Clarify meaning without changing it
Add related resources or links
Always respect the author’s intent
Don’t use edits to reply to the author

create code fences with backticks ` or tildes ~
```
like so
```
add language identifier to highlight code
```python
def function(foo):
print(foo)
```
put returns between paragraphs
for linebreak add 2 spaces at end
_italic_ or **bold**
quote by placing > at start of line
to make links (use https whenever possible)

<https://example.com>

[example](https://example.com)

<a href="https://example.com">example</a>
MathJax equations $\sin^2 \theta$

formatting help »
answering help »

MathJax help »