AbdurRakib wrote:

The light in a restroom operates with a 15-minute timer that is reset every time the door opens as a person goes in or out of the room.Thus,after someone enters or exits the room,the light remains on for only 15 minutes unless the door opens again and reset the timer for another 15 minutes.If the times listed above are the times at which the door opened from 8:00 to 10:00 ,approximately how many minutes during this two-hour period was the light off ?

A. 10

B. 25

C. 35

D. 40

E. 70

We are looking for two consecutive times (when the door opened) with a gap of more than 15 minutes, since only then the light would be off.

Since the door is opened at 8:31, the light shuts off at 8:46 and stays off until 8:54, so the light is off for 8 minutes.

Since the door is opened at 9:11, the light shuts off at 9:26 and stays off until 9:29, so the light is off for 3 minutes.

Finally, since the door is opened at 9:31, the light shuts off at 9:46 and stays off at least until 10:00, so the light is off for 14 minutes (at least until 10:00).

Thus, the light was off for a total of 8 + 3 + 14 = 25 minutes.

Answer: B

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