11. During an experiment a buzzer beeped once every quarter hour. If the first beep was at 3:15, what was the time at the 18th beep?
Answer: D
The time at the 18th beep is 7:30.
The buzzer beeping once every quarter hour means it beeps every 15 minutes. Starting from the first beep at 3:15, the time at the 18th beep can be calculated by adding 15 minutes for each subsequent beep.
A) 6:30
This option is incorrect because it would imply that only 15 beeps occurred after 3:15, which would only take us to 6:00 (3:15 + 15 minutes x 15 beeps = 6:00). Therefore, this does not account for the full 18 beeps.
B) 7:00
This option is also incorrect as it suggests that the 18th beep occurred at 7:00. Adding 15 minutes for 17 beeps after the first beep at 3:15 results in 6:45, which is not the time for the 18th beep.
C) 7:15
This choice is incorrect because it implies that the 18th beep is only 15 minutes after the 17th beep at 7:00. However, the correct calculation shows that the 18th beep occurs at 7:30 instead.
D) 7:30
This is the correct choice. Starting at 3:15 and adding 15 minutes for each of the 18 beeps (which corresponds to 17 intervals of 15 minutes) brings us to 7:30. The calculation is as follows: 3:15 + (17 x 15 minutes) = 7:30.
E) 0.322916667
This option is incorrect as it appears to represent a decimal rather than a specific time. It does not correspond to any time format that would be relevant to the question regarding the buzzer beeping at quarter-hour intervals.
Conclusion
The correct answer, 7:30, is derived from the accurate addition of 15-minute intervals starting from 3:15 for a total of 18 beeps. All other options fail to reflect the correct total time elapsed, demonstrating a misunderstanding of the time calculation involved.