Democratic strategists are predicting $($well$,$ praying for$)$ a good economy in $2024.$ They are looking at the following probabilities: $50\%$ chance that there will be a boom, $40\%$ chance that the economy will be flat or neutral and only a $10\%$ chance of a recession. The expected returns on Ruppel Intl. shares are $15\%$ in the event of a boom, $10\%$ if the economy's flat and $-20\%$ in a recession. What's the standard deviation of these returns?