急!!兩題probability問題!!求高手指教

http://cck829.webs.com/photos/IMG-20121111-WA0001.jpg Q2 An urn contains 12 blue balls, 22 yellow balls, 12 red balls and 54 green ball. Give your answers correct to 4 decimal places.

a) Suppose 7 balls are drawn randomly each at a time with replacement from the urn. Find the following probabilities:i) Getting exactly one chosen ball is red. ii) Getting all seven chosen balls are not blue. iii) Getting at most two chosen balls are yellow. iv) Getting at most two chosen balls are

b) Balls are randomly selected one by one from the urn until three green balls are drawn. Find the probability that exactly ten balls have to be selected in this process.

Q3 The hourly incomes of workers in the industry are assumed to follow the normal distribution with a mean of $34.7 and a standard deviation of $5.8. Give your answers correct to 1 decimal place in part (a) and 4 decimal places in parts (b) to (d) respectively.

(a) If there are 24.2% of workers in the industry with hourly incomes less than $K, find the value of K. (b) Find the probability that a randomly selected worker in the industry with hourly income less than $38.

(c) If a random sample of 10 workers in the industry is selected, find the probability that the average hourly income of these workers is between $30 and $34.

(d) Suppose a random sample of 8 workers the industry is selected. Find the probability that at most 3 workers with hourly incomes less than $34 are selected given that at least 4 workers with hourly incomes greater than or equal to $34

請問Q2 a) 點解唔可以咁計: (88C6)*(12C1) / (100C7)