計算機會率

qwertyuiop35920005 's answer is not comprehensive
I think the value 2.33 is meaningless, if it is calculated by 70% / 30%

Actually, if you want to find out the expected no. of gamble to win 1 time, you should use Binomial Distribution
http://en.wikipedia.org/wiki/Binomial_distribution
the probability to win = p = 0.7
the probability to lose = q = 1 - p = 0.3
let say we have n trials, the expected no. of win = np = 0.7n
now, you want to know how many trials to have 1 win
so, 0.7n = 1
n = 1.4286 (but not 2.33)
For theoretical background of Binomial Distribution of Binomial Theorm, please refer to Wikipedia
Actually, Binomial Theorm is widely used for the calculation of Option Pricing in Stock Market or Derivatives Market in the real finanical world.

2009-06-06 23:40:04 補充：
assume S = X1 + X2 + ... ... + Xn
assume Xi is the cash flow in the ith gamble
assume n is a constant
assume Xi are independent

2009-06-06 23:43:23 補充：
E(S) = E(X1) + E(X2) + ... ... + E(Xn) = n * E(Xi) = n * (0.7 * 500 - 0.3 * 1500) = -HK$100n < 0

Hence, we could conclude that the gamble is not fair

For a fair game, the E(S) should be = HK$0

qwertyuiop35920005 講咩慢左３７秒呀..

= =
不就是７０除３０嗎－　－？
囉

另外７０％　ｘ＄５００＝羸
　　３０％　ｘ＄１５００＝輸

羸的奇望值由成本高，收益少
所以輸唔過

2009-06-04 23:18:40 補充：
慢左３７秒ｔｉｍ－　－