Akono lists some of the variables in his investigation to answer the question: How does the mass of salt affect the speed of rusting? Variables:
- amount of salt
- how much of the nail has gone rusty?
- type of nail
- time nail is left in test tube
--From the list of variables above, identify the variable Akono should change.

Amount of salt

Explanation:

Since he is testing the speed of rusting depending on the mass of salt, the variable he should change in the experiment is the amount of salt.

The type of nail and the time the nail is left in the test tube should not change in order to keep the experiment fair. How much of the nail has gone rusty is a dependent variable and depends on the amount of salt.