Editing Rule of 72 (section)

==History==
An early reference to the rule is in the ''[[Summa de arithmetica]]'' (Venice, 1494. Fol. 181, n. 44) of [[Luca Pacioli]] (1445–1514). He presents the rule in a discussion regarding the estimation of the doubling time of an investment, but does not derive or explain the rule, and it is thus assumed that the rule predates Pacioli by some time.
{{cquote|'' A voler sapere ogni quantità a tanto per 100 l'anno, in quanti anni sarà tornata doppia tra utile e capitale, '''tieni per regola 72''', a mente, il quale sempre partirai per l'interesse, e quello che ne viene, in tanti anni sarà raddoppiato. Esempio: Quando l'interesse è a 6 per 100 l'anno, dico che si parta 72 per 6; ne vien 12, e in 12 anni sarà raddoppiato il capitale.'' (emphasis added).}}
Roughly translated:
{{cquote|In wanting to know of any capital, at a given yearly percentage, in how many years it will double adding the interest to the capital, '''keep as a rule [the number] 72''' in mind, which you will always divide by the interest, and what results, in that many years it will be doubled.  Example: When the interest is 6 percent per year, I say that one divides 72 by 6; 12 results, and in 12 years the capital will be doubled.}}