In many cases, the portfolio return is at least approximately normally distributed. Then Excel's NORMDIST function can be used to calculate the probability that the portfolio return is negative. The relevant formula is $=$ NORMDIST$(0,$ mean, stdev,True$),$ where mean and stdev are the expected portfolio return and the standard deviation of portfolio return, respectively.

a$.$ Modify the model in Example $7\mathrm{.}9$ slightly, and then run Solver to find the portfolio that achieves at least a $0\mathrm{.}12$ mean return and minimizes the probability of a negative return. Do you get the same optimal portfolio as before? What is the probability that the return from this portfolio will be negative?

b$.$ Using the model in part a$,$ proceed as in Example $7\mathrm{.}9$ to use SolverTable and create a chart of the efficient frontier. However, this time, put the probability of a negative return on the horizontal axis.