Final answer:
The cell phone battery depletes at a rate of 8% per hour, and it will take a total of 12.5 hours from a full charge at 7 am to reach a 0% charge, which occurs around 7:30 pm.
Step-by-step explanation:
To find out how many hours it takes for the cell phone battery to deplete from a full charge to 0%, we need to determine the rate at which the battery charge is decreasing. We know from the student's question that at 7 am the battery is fully charged (100%), and by 11 am it is at 68% charge. This means that in 4 hours, the battery has used up 32% of its charge.
We calculate the rate of charge depletion per hour by dividing the percentage of charge depleted by the time elapsed: 32% charge ÷ 4 hours = 8% per hour.
To determine when the battery will be completely depleted, we divide the remaining charge at 11 am by the rate of depletion: 68% charge ÷ 8% per hour = 8.5 hours.
Since these 8.5 hours are counted from 11 am, we add this time to 11 am to find the time at which the battery would be at 0%. 11 am + 8.5 hours = 7:30 pm. So, the total time from a full charge at 7 am to 0% charge would be 12.5 hours.