Chapter 3 - Section 3-4 - Mean and Variance of a Discrete Random Variable - Exercises - Page 77: 3-60

$μ=11/7$, $σ²\approx0.531$

Given, $P(x=1)=4/7,P(x=2)=2/7,P(x=3)=1/7$, then, $μ=E(x)=∑xP(x)=1\times\frac{4}{7}+2\times\frac{2}{7}+3\times\frac{1}{7}=11/7$ $σ²=V(x)=∑x²P(x)−μ²=1²×\frac{4}{7}+2²×\frac{2}{7}+3²×\frac{1}{7}-\frac{11}{7}^{2}=26/49\approx0.531$