* Ice cube made of just water: This will melt at the standard freezing point of water (0 degrees Celsius or 32 degrees Fahrenheit).

* Ice cube made of salt and water: This will actually melt slower than the plain water ice cube. Here's why:

* Salt lowers the freezing point of water. This means the salt water ice cube will need to get colder than 0 degrees Celsius to freeze completely.

* Salt disrupts the formation of ice crystals. This makes it harder for the ice to form in the first place and makes the resulting ice less dense.

* Ice cube made of sugar and water: This will also melt slower than the plain water ice cube.

* Sugar also lowers the freezing point of water, though not as dramatically as salt.

* Sugar also disrupts the formation of ice crystals in a similar way to salt.

In summary:

The plain water ice cube will melt the fastest, followed by the sugar water ice cube, and the salt water ice cube will melt the slowest.