A portfolio is composed of two stocks, A and B$.$ Stock A has a standard deviation of return of $35\%,$ while stock B has a standard deviation of return of $15\%.$ The correlation coefficient between the returns on A and B is $\mathrm{.}5.$Stock A comprises $40\%$ of the portfolio, while stock B comprises $60\%$ of the portfolio. The standard deviation of the return on this portfolio is $\_\_\_\_\_\_\_\_\_.$