probability
posted by padma.c .
A random experiment of tossing a die twice is performed. Random variable X on this
sample space is defined to be the sum of two numbers turning up on the toss. Find the
discrete probability distribution for the random variable X and compute the corresponding
mean and standard deviation.

36 possibilities
number of times sum
1 * 2 = 2
2 * 3 = 6
3 * 4 = 12
4 * 5 = 20
5 * 6 = 30
6 * 7 = 42
5 * 8 = 40
4 * 9 = 36
3 * 10= 30
2 * 11= 22
1 * 12= 12

36 and 252
mean = 252/36 = 7
go ahead and compute sigma 
A dictionary contains 80,000 words, of which a certain student knows 30,000. If 100 of the 80,000 words are selected at random from the dictionary without replacement, find the probability:the student knows exactly 40 of the 100 words