Ashley wants to form a risky portfolio by investing half of her money in stocks and half in bonds. She puts half in a stock mutual fund that has an expected return of 18% and a standard deviation of 23%; and the rest in a corporate bond mutual fund that has an expected return of 14% and a standard deviation of 17%. The stock and bond funds have a correlation of -0.15. What is the standard deviation of Ashley's portfolio? *(please provide an explained answer)