明天考，两道数学题不会，急求！！！！

yang8630

都是prep 2上的

1）To celebrate a colleague's retirement, the T coworkers in an office agreed to share equally the cost of a catered lunch. If the lunch costs a total of x dollars and S of the coworkers fails to pay their share, which of the following represents the additional amount, in dollars, that each of the remaining coworkers would have to contribute so that the cost of the lunch is completely paid?
我怎么算都是X/T  但答案给的是 SX/T（T-S）

2) If a coin has an equal probability of landing heads up or tails up each time it is flipped, what's the probability that the coin will land heads up exactly twice in 3 consentive flips? 0.375

MRFROG

1)原先每个人付X/T,现在每个人付X/(T-S),那么现在比原先多付X/(T-S)-X/T=SX/T（T-S）

2)用贝努力公式就可得到C(2,3)*(0.5^3)，如果不明白贝努力，那就排列组合，罗列出以下几种情形

正正反

正反正

反正正

分别算出每种的概率，然后相加，即得答案0。375

yang8630

哈哈，不知道当时为什么懵了   多谢了