Please provide working / formula

The next year's return for Apple Ltd depends on the state of next year's economy. The return is predicted to be 20% in a boom, 6% in average conditions, and -3% (i.e. minus 3%) in a contraction. The probability of these outcomes is 20% chance of a boom, 50% chance of average conditions, and 30% chance of a contraction. Calculate the standard deviation of expected returns for Apple Ltd based on this data.