what formula is applied, I mean what method
A century is 100 years. 4 times 100 equals 400. The answer is 400
Doubling every 3 years, meaning 200% every 3 years.


The answer is 24 years.
Hope this helps. - M
Why not? Because every math system you've ever worked with has obeyed these properties! You have never dealt with a system where a×b did not in fact equal b×a, for instance, or where (a×b)×c did not equal a×(b×c). Which is why the properties probably seem somewhat pointless to you. Don't worry about their "relevance" for now; just make sure you can keep the properties straight so you can pass the next test. The lesson below explains how I kept track of the properties.