In practice excercise 7.2, we are asked which stock in not on the efficient frontier. I believe Stock C is not on the efficient frontier, because a combination of 2 other stocks can be formed with the same expected return and a lower standard deviation.

My question is - what is the best way to figure out this out quickly? (I'm assuming we aren't limited to combing only 2 stocks, and there are various proportions we could assign them) -

How should we go about trying to figure out the answer to this quickly, for example on the final exam?

[NEAS: No formula works for all cases. The final exam questions are simple enough that you can solve them by first principles.]