Like a good economist, you calculated the opportunity cost of getting your college degree. suppose that at your university, you will pay $12000 each year for tuition, $3000 each year for textbooks, and $12000 per year for room and board. before you left for college, your boss at your high-school job offered you a job paying $15000 per year. assume that if you decided not to go to college, your parents would not let you live at home. what is your opportunity cost for four years of college?

Opportunity cost of going to college is actually the salary forgone. 
With a yearly salary of $15,000 and a rent of $12000 yearly (parents would not let you live at home), we are left with $15000 - $12000 = $3000 per year 
For 4 years, the opportunity cost will be 4 x $3000 = $12000

Secured loans are protected by collateral security. examples: title loans where the borrower uses the car title as collateral security interest rates are lower and repayments are longer therefore risk is low as a collateral security is given as guarantee to the bank in case of failure to pay the loanunsecured loans are not protected by collateral security but it is given on the creditworthiness of the borrower, trusting that she/he will pay on time. examples; payday loans is a short term loan where the borrower lends money from the bank and pays is back when the borrower receives his/her wages.interest rates are high and repayments are shorter. but the risk is high as no collateral security is given as a guarantee to the bank for payment of the loan