An investor can design a risky portfolio based on two stocks, A and B. Stock A has an expected return of 15% and a standard deviation of return of 16%. Stock B has an expected return of 11% and a standard deviation of return of 4%. The correlation coefficient between the returns of A and B is 0.50. The risk-free rate of return is 7%. The proportion of the optimal risky portfolio that should be invested in stock A is............