if the first term of an infinite geometric series is equal to twice of the sum of all the terms that follows it, then the value of r is?

1/3, 1, 1/2 or 2? how?

assuming that the common ratio, r, satisfies -1<r<1, the sum of the series is a/(1-r), where a is the first term of the series.
 
if a = 2[a/(1-r) - a], then a = 2a/(1-r) - 2a
 
                                     3a = 2a/(1-r)
 
                                     3a - 3ar = 2a
 
                                         - 3ar = -a
 
                                              r = 1/3