1. The question asks why we use 8 years in part (a). 2. Usually, the number of years corresponds to the time period given or required in the problem. 3. For example, if the problem states or implies a duration of 8 years, we use that value in calculations. 4. This is important because time affects growth, decay, interest, or other time-dependent processes. 5. Without the exact problem statement, the best explanation is that 8 years is the specified or relevant time frame for the calculation in part (a).