The elevator in an eleven-storied office building travels at the rate of one floor per 1/4 minute, which allows time for picking up and letting down passengers. At the main floor and at the top floor the operator stops for 1 minute. How many complete (return) trips will the operator make during a 7 hours period?

Going up from the main floor to the top floor:

• Time to reach the top floor: 10 floors * (1/4 minute per floor) = 10 * (1/4) = 2.5 minutes
• Stop at the top floor for 1 minute
• Total time for the upward trip: 2.5 minutes + 1 minute = 3.5 minutes

Going down from the top floor to the main floor:

• Time to descend from the top floor to the main floor: 10 floors * (1/4 minute per floor) = 10 * (1/4) = 2.5 minutes
• Stop at the main floor for 1 minute
• Total time for the downward trip: 2.5 minutes + 1 minute = 3.5 minutes

So, each complete round trip (going up and coming down) takes a total of 3.5 + 3.5 = 7 minutes.

Now, let's calculate how many complete round trips the operator can make in a 7-hour period (which is 7 * 60 = 420 minutes):

Number of complete round trips = Total time available / Time per round trip Number of complete round trips = 420 minutes / 7 minutes per round trip = 60

So, the elevator operator can make 60 complete round trips during a 7-hour period.