Let X be a random variable whose possible values `x_1, x_2, x_3, ..., x_n` occur with probabilities `p_1, p_2, p_3,..., p_n`, respectively. The mean of X, denoted by µ, is the number `sum_(i=1)^n x_i p_i` i.e. the mean of X is the weighted average of the possible values of X, each value being weighted by its probability with which it occurs.
The mean of a random variable X is also called the expectation of X, denoted by E(X).
Thus , E(X) = µ = `sum_(i = 1)^n x_ip_i = x_1p_1 + x_2p_2 + ... + x_np_n.`