the formula to get the expected value is
[tex]E(x)=\sum ^n_{i\mathop=1}x_if(x_i)[/tex]So, first we calculate teh probabilities. We have 20 bills in total. so we divide the quantity of bills of each value by the total:
we have 8 ones: 8/20= 2/5
5 fives: 5/20=1/4
4 tens: 4/20= 1/5
3 twenties = 3/20
now, we multiply each value with te probabilitiy of picking it and add all
[tex]E=\text{ 1}\cdot\frac{2}{5}+5\cdot\frac{1}{4}+10\cdot\frac{1}{5}+3\cdot\frac{3}{20}=\text{ 4.1}[/tex]the expected value is 4.1
