The <em>opportunity cost</em> is a <em>potential gain you lose by choosing one particular alternative over the other</em>. It is the highest benefit of another alternative that you did not choose.
Let us look at the <u>given example</u>. Say, you earn 10 dollars per hour. It means that you need to work for three hours to be able to buy new clothes for 30$. Meanwhile, you do not enjoy your summer job and would be much happier to spend these three hours hiking with your dog. <u>The joy you have lost</u> by choosing the alternative of earning these 30$ to be able to buy new clothes, is the opportunity cost.
I would attend the university without being know so. Because I would a fully get the knowledge I need rather then to be given something without the knowledge and I would feel cheated.