Baseball betting and probablity
Your favorite baseball team is playing against your uncle's favorite team in the World Series. At the beginning of each game, you and your uncle bet on the game's outcome. Your uncle, being wealthy and carefree, always lets you choose the amount of the bet. If you bet b dollars and your team wins the game, your uncle gives you an IOU for b dollars. But if they lose the game, you give him an IOU for b dollars. When the series is over, all outstanding IOUs are settled in cash. You would like to walk away with \$100 in cash if your team wins the series, and lose \$100 if your team loses the series. How much should you bet on the opening game? (For non baseball fans, the first team to win a total of four games wins the series).
Can somebody please explain how did they get value 31.25 using binomial tree.