Wikipedia:Reference desk/Archives/Mathematics/2016 April 14

= April 14 =

What's the name for this?
I learned this... thing back in high school algebra. As an adult, I use it nearly every day to figure out proportional questions when solving for x. It's the single most useful thing I learned in high school math.

For example, a 3D render has completed 7% in 9 minutes. How many minutes will it take to reach 100% completion?

$7 minutes⁄9%$ = $x minutes⁄100%$      x = 78 minutes

or one side of an image is 825 pixels at 100%, how big do I have to scale it to enlarge to 1080 pixels?

$825 pix⁄100%$ = $1080 pix⁄x%$    x = 131%

Is there a common name for this? --Navstar (talk) 18:51, 14 April 2016 (UTC)


 * See cross-multiplication. Gandalf61 (talk) 18:54, 14 April 2016 (UTC)


 * Cross-multiplication 175.45.116.59 (talk) 23:45, 14 April 2016 (UTC)


 * Note that in your first example, you've exchanged the minutes and percentage values when doing the calculation, so the answer should be x = 129 minutes→86.136.6.197 (talk) 09:59, 17 April 2016 (UTC)
 * I don't think your answer is correct. --Navstar (talk) 19:15, 14 December 2016 (UTC)
 * They are not saying that your technique is incorrect, but only that there is a careless error or a typo somewhere in that first question. You wrote "7% in 9 minutes", but in the fraction you used 7 minutes and 9%.  Which is it, 9 minutes or 7 minutes?  If it is 7% in 9 minutes then it clearly takes more than a minute for each 1%, so it will take over 100 minutes for 100%, and using your method will show that it takes 128.57 minutes which they rounded up to 129 minutes.  If there is a typo in your question and you meant to write "7 minutes for 9%", then your formula and answer is correct. -- ToE 07:55, 21 December 2016 (UTC)

The name is reguladetri or regula de tribus. Bo Jacoby (talk) 21:09, 17 April 2016 (UTC).