An investor is analyzing the risk of a possible investment by producing three different scenarios.Under a pessimistic scenario, the property would produce a BTIRRp of 8%; a most-likely scenario produces a BTIRRp of 12%.The investor assigns the pessimistic scenario a 25% chance of occurring, the most-likely case a 60% chance of occurring, and the optimistic scenario a 15% chance of occurring.What is the standard deviation of the returns? the answer is 0.000156 but I am not sure about how to calculate it.