please explain how to do these

Review What is the historical average return on a stock with the following returns over the last 4 years: 6%, 15%, -10%, -20%? (Answer as a percent. Enter only numbers and decimals in your response.) Numeric Answer: Correct Answer Correct Answer 1.75 Historical Returns 7 - Calculations Review What is the standard deviation for a stock with the following returns over the last 4 years: 6%, 15%, -10%, -20%? (Answer a a percent. Enter only numbers and decimals in your response.) Numeric Answer: Correct Answer 13.18 3.18