मान लीजिए X एक यादृच्छिक चर (random variable) है जो एक निश्चित धनात्मक पूर्णांक n के लिए मान 1, 2, ..., n लेता है। यदि 1 ≤ k ≤ n के लिए P(X = k) = 1n है, तो X का प्रसरण (variance) है:

A box contains 'x + 4' red, 'x – 1' green and 'x – 5' blue balls. If two balls are drawn from the bag one after another without replacement, then the probability of getting one red and one green ball is equal to the probability of getting a sum equal to or more than 8 when two fair dice are rolled. Find the probability of drawing a red ball when one ball is drawn at random from the bag.

Four persons are chosen at random from a group containing 3 men, 2 women, and 4 children. The probability that exactly two of them are children is: