What is the best currency to take to Kenya?

How long does it take for old U.S. currency to circulate back to the Federal Reserve globally, vis a vis the printing of a new bill (e.g. the new $100)?

• There are some cool thought experiments that come with this. From the perspective of a not-econ-savvy individual, a glance at the factors involved for a given country yields the following: -How widely accepted U.S. currency in that nation -Whether or not the nation's currency is backed by gold or its government's full faith and credit -How likely individuals are to keep it in cash as opposed to putting it in their bank (in which case it is exchanged for the national currency and heads back to the United States) -The nation's key industries (cash crops, e.g.) The reason I ask this is because of feedback loops. I'm curious to know if there would ever be a situation where U.S. currency circulates in such tight circles, in cash, and away from any system that would suck it back into the United States. How does it vary by country, moreover? (This question is potentially a data scientist's dream!)

• Answer:

  For an interesting sidenote to this question (but not an answer) see Where's George? http://www.wheresgeorge.com