An ice cube with salt on it will melt first. This is because salt lowers the freezing point of water. In other words, it takes more energy for water to freeze when there is salt dissolved in it. When you add salt to an ice cube, the salt molecules interfere with the formation of ice crystals, which causes the ice to melt at a lower temperature.

The amount of salt required to melt ice depends on the temperature of the ice. For example, if the ice is at 32°F, you will need to add about 1/4 teaspoon of salt per cup of water. If the ice is at a lower temperature, you will need to add more salt.

The process of melting ice with salt is called freezing point depression. This process is used in a variety of applications, such as keeping roads and sidewalks clear in winter and preserving food.