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It takes $6$ technicians a total of $10$ hours to build a new server from direct computer, with each working at the same rate. If six technicians start to build the server at $11:00$ am, and one technician per hour is added beginning at $5:00$ pm, at what time will the server be completed?

1. $6:40$ pm
2. $7:00$ pm
3. $7:20$ pm
4. $8:00$ pm

To complete the job 6*10=60 man/hours are needed.

By 5:00 PM 6*6=36 man/hour done. 24 is left.

5:00 PM - 6:00 PM 7 man/hour;    ->36+7=43

6:00 PM - 7:00 PM 8 man/hour;    ->43+8=51

7:00 PM - 8:00 PM 9 man/hour.    ->51+9=60(work completed)

Hence,Option(D)8:00 pm.
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