NO.PZ2018062016000094
问题如下：
Using one-period binomial model, Jack calculated that the terminal value of a stock is $55, if the current price is $50, the size of an up move is 1.25 and a down move is 0.8. The probability of a down move that Jack picked is:
选项：
A. 0.50
B. 0.67
C. 0.33
解释：
C is correct. The probability of an up move is p, then the probability of a down move is (1-p). Draw the equation that (p)uS + (1 – p)dS = (p)×50×1.25+ (1 – p)×50×0.8 = 55. Solving the equation, p = 0.67, so (1-p) = 0.33.
为什么这里的size move up要理解成1.25倍？英文的意思是涨1.25元啊
