Answer:
The opportunity costs of letting your colleague to extend the loan for another month are:
Your total opportunity cost depends on what you actually decide to do with the money, if you invest all of it on the oil-well venture it is $1,428, or all of it in the new IT stock it is $2,295, but if you invest 50/50 on each, then the opportunity cost would be $1,861.50, or any other possible combination.
Opportunity costs are the extra costs or benefits lost from choosing one investment or activity over another alternative.