# How would I have a portfolio with a security of a weight greater than 1?

I recently started learning about finance and we talked about Markowitz Portfolio Theory. I understand that a portfolio consists of different assets, each of them with a certain weight. So, for example, I could have a portfolio with 50% Exxon Mobil and 50% IBM.

However, we then mentioned that if we are allowed to short IBM, for example, we could end up with a portfolio of "weights" 120% Exxon Mobil and -20% IBM. Then, the expected return calculation would yield 1.2 * (Expected return of Exxon) - 0.2 * (Expected return of IBM).

I am a little confused how this calculation makes sense. How could I have more than 1.0 for a weight of a portfolio. My professor mentioned something about using the short to "lever up" investment in Exxon, but I'm not quite sure how to make senes of that.

Here's what your professor means by "leveraging" (we'll skip efficient frontier and other portfolio theories). Suppose you really think that Exxon stock will go up but only have $10,000 to invest. You can borrow$10,000 ("short" cash) and buy $20,000 in Exxon stock. You have "200%" in Exxon stock and -100% in cash. If Exxon goes up 10% to$22,000, your gain is $2,000 which is a 20% return on your$10,000 investment. Your return has been multiplied by a factor of 2. Leverage works both ways, though - if the stock goes down 10%, you'll lose 20%, so you can lose all of your investment if the stock goes down just 50% (the other 50% will need to be sold to pay back the loan).
• The mathematical version of this answer is 120% + -20% = 100%. – RonJohn Jun 20 at 21:21