Probability of Winning

#1
Hi need some help to understand how the below probability can be determined

If I pay $100 to play a game where if lose i lose the $100 but in case i win I receive $300 from the dealer. What is the probability of winning if the game is a zero sum game(fair game)
 
#2
In a zero-sum game, your expected winnings over the long term (i.e. many trials) are zero. Do you know how to calculate the expected winnings?

E = Pwin×AmountwinPlose×Amountlose.

You also know that Pwin + Plose = 1.

That is enough info to solve the problem.