It's actually a little difficult to answer with certainty without running an experiment. When ice melts in water, heat from the water is being transferred, or conducted, to the ice until the ice warms up enough to melt into water. This process cools down the water immediately around the ice, and so more heat will be conducted in from the surrounding water, and some of the water will move because cold water is slightly denser than warm water.

Heat flow in a stationary medium is influenced by the thermal conductivity of the material. "The higher the thermal conductivity is, the faster heat can flow, and the faster the ice will melt." According to this site the thermal conductivity of saltwater decreases as salt is added, which suggest that the saltwater will melt the ice more slowly.

But water is not stationary, as it cools down it gets denser and falls away from the ice, so warm water can flow in without the heat flowing through the cold water. This process is called heat convection.

Saltwater changes in density a bit faster as it cools down than freshwater which means it might move faster when you put the ice cube in, which helps melt the ice, but it also is more viscous so it's harder, which makes it harder to move which slows down the melting process. This was found out from procedures.

Since there are two factors working in opposite directions I don't know whether there will be more or less convection in the saltwater vs. freshwater. This could be calculated but it's tricky. If there is less convection in saltwater then it will almost definitely take longer to melt the ice than freshwater. If there is more convection in salt water then the time to melt the ice may be closer, or the saltwater may even melt it faster.

My guess would be that the higher thermal conductivity of the freshwater will melt the ice more quickly regardless of convection. But I think the best way to find out is to run an experiment. Measure out a quantity of salt and water and combine them. Keep track of how much salt there is, so you can compare saltwater with different salt concentrations. You can keep track of it by weight with a kitchen scale or by volume with measuring cups and spoons. Fill two identical glasses with the same volume of water, one saltwater, one tap water, and put in two identical ice cubes from the same batch of ice. Time how long it takes each ice cube to melt. For fun, see if you get different results with different concentrations of salt but the same volume of water, what happens when you change the volume of water, or what happens if you try using distilled water instead of tap water.

Long answer but hopefully it answers the question!

[Note from one of our scientists who wrote answers related to this question : The following answers (#2 and #3) relates to melting point depression due to solutes. Please read the Answer #1 on this link in order to enrich the answer that you need. ScienceLine Moderator.]

An ice cube will melt faster in fresh water. This has to do with something called "melting point depression." The melting point is the temperature at which your substance will melt. In this case, the substance we're interested in is water, since ice cubes are the solid form of liquid water.

When you add salt to water, it lowers its melting point. This lowering of melting point is called "melting point depression." That means that any solid water we add will only melt at a lower temperature, and the ice cube we add will melt slowly compared to the fresh water case.

Try it at home! Take two glasses and fill them with water. In one of your cups, add a tablespoon of salt and stir. Then take two ice cubes from your freezer and put them in at the same time. See which one melts faster! :)

Assuming equal temperature, ice melts faster in salt water because salt water has a lower freezing point than fresh water, so the ice cube will have to absorb less heat in order to melt in salt water than in fresh water.