Final answer:
Ellen should calculate her travel time to work, establish a 90th percentile buffer for variability, and subtract this total time from her 9:00 am arrival time to determine the latest departure time that allows her to be on time at least 90% of the occasions.
Step-by-step explanation:
The student's question asks how late Ellen can leave her home to ensure she arrives at work by 9:00 am while maintaining a 90% punctuality rate. To answer this, we need to calculate the amount of time it takes Ellen to travel to work and then determine the latest departure time that satisfies the requirement of being on time at least 90% of the time.
Step 1: Identify the usual time it takes for Ellen to travel to work and any potential variables that may affect this time, such as traffic conditions.
Step 2: Determine the 90th percentile of the travel time if given, or establish a reliable buffer time that Ellen should aim to leave before her usual departure time to ensure punctuality.
Step 3: Calculate the reverse schedule by subtracting the travel time, including the buffer, from the arrival time (9:00 am). For example, if Ellen's trip takes 30 minutes and the buffer is 10 minutes, she should leave by 8:20 am to be on time in 90% of the cases.
Remember to plan your route and consider practicing the commute in advance to ensure that the time estimates are accurate, helping to prevent the possibility of being late.