I know this is easy, but i having a mental blank here… a stock has a 40% chance of earning 10%, a 40% chance of earning 12.5% and a 20% chance of earning 30%. what is its std deviation of expected returns?

mean = 0.4*10%+0.4*12.5+0.2*30%=4+5+6=15 st.dev=sqrt[0.4(10-15)^2+0.4(12.5-15)^2+0.2(30-15)^2]=sqrt[10+2.5+45]=7.5

ah thanks, for some reason I kept thinking I had to square the weights…