The gambler playing a fair game (with 0.5 probability of winning) will eventually either go broke or double his wealth. Let's define that the game ends upon either event. These events are equally likely, or the game would not be fair. So he has a 0.5 chance of going broke before doubling his money. If he doubles his money, a new game begins and he again has a 0.5 chance of doubling his money before going broke. After the second game there is a 1/2 x 1/2 chance that he has not gone broke in the first and second games. Continuing this way, his chance of not going broke after n successive games is 1/2 x 1/2 x 1/2 x . . . 1/2^n which approaches 0. His chance of going broke after n successive games is 0.5 + 0.25 + 0.125 + . . . 1/2^n which approaches 1.
