The primary waves reached Chicago at AM when did the earthquake occur in Chicago time what math operation did you use to determine he time of the earthquake?

To determine the time of the earthquake in Chicago time, you would subtract the travel time of the primary waves from the time they reached Chicago. For example, if the primary waves reached Chicago at 10:00 AM and the waves took 30 minutes to arrive, you would perform the operation: 10:00 AM - 0:30 (30 minutes) = 9:30 AM. Thus, the earthquake occurred at 9:30 AM Chicago time.