Suppose that a meeting you are attending ends $15$ minutes earlier than scheduled. You are considering either going for a quick walk in the park close to your workplace, or working on a business project, or chatting with a colleague. Suppose that the walk costs you nothing and you value it at $$25$; working on the business project costs you $$10$ and you value the completed work at $$30$; and chatting with colleagues costs you nothing and you value the interaction at $$15.$ What is the opportunity cost of chatting with colleagues during the "unexpected free $15$ minutes"?