Brewed coffee is often too hot to drink right away. You can cool it with an ice cube, but this dilutes it. Or you can buy a device that will cool your coffee without dilution - a 150g aluminum cylinder that you take from your freezer and place in a mug of hot coffee.
If the cylinder is cooled to -20?C, a typical freezer temperature, and then dropped into a large cup of coffee (essentially water, with a mass of 500 g) at 85?C, what is the final temperature of the coffee?