$"$An investor can design a risky portfolio based on two stocks, A and B$.$ Stock A has an expected return of $9\%$ and a standard deviation of return of $7\%.$ Stock B has an expected return of $5\%$ and a standard deviation of return of $6\%.$The correlation coefficient between the returns of A and B is $-0\mathrm{.}5.$ The risk$-$free rate of return is $3\mathrm{.}5\%.$ The proportion of the optimal risky portfolio that should be invested in stock B is approximately $\_\_\_\_\_\_\_\_\_.$ Note: Express your answers in strictly numerical terms. For example, if the answer is $5\%,$ write $0\mathrm{.}05"$