You go to cinema and find that ticket window is closed.

There is a half an hour to start the movie and you are committed to watch the movie.

You are having Rs. 15/- in your pocket. and there is a person who intends to sell his ticket of Rs. 10/- in Rs. 20/-.

You would request seller to give you the ticket of Rs. 10/- @ Rs. 15/-. Now it depends upon the seller to sell ticket at Rs. 15/- or wait for another buyer at 20/-.