Radioactivity is a phenomenon in which unstable atoms decay, emitting radiation and transforming into another element. Radioactive elements have an inherent instability that causes them to naturally break down over time. This process is known as radioactive decay. The rate of decay for any given element is characterized by its half-life, which is the amount of time it takes for half of the original amount of the element to decay.

In this case, we are looking at a certain radioactive element with a half-life of 5 hours. With this information, we can determine how much of the original mass will be left after 2 days (48 hours). To do so, we need to calculate how many half-lives have passed during that time period: 48 hours / 5 hours = 9.5 half-lives.