If you don't have blocks of ice ready-bought, then your first step should be to pop some water into the freezer and to make the ice. They should be approximately the same size.
Once you have the ice ready set them in separate clear plastic containers, side by side. Label these containers with the kind of salt that will be added to it.
Sprinkle an even amount of each kind of salt in each separate container. Now, you'll just have to wait to see what happens! 10-minute intervals are good times to check on the ice.
Observe the progress of the melting ice and record your findings until all the ice has melted.
Record your results. How fast did it take for all the ice to melt out of each container?