[Leica] Need help with formulae

[leica at web-options.com (Bob W)]

how can a number double .n of a time?

Bob 

> 
> G'day all,
> 
> does anyone know a formula which allows you to calculate a 
> value in a geometric progression. For example if a number 
> doubles 2.8 times in 10 years, what will that number be. At 
> the moment the only way I can do it is "long hand" ie if 10 
> is going to double 2.8 times then it would be 10 doubled to 
> 20, twenty doubled to 40 then add 80% of the next double ie 
> 32 plus the 40 equals 72, BUT is there a formula, so I can 
> put in other times of doubling. 
> 
> Troubled
> 
> Alastair