Lisa, an experienced shipping clerk, can fill a certain order in 13 hours.

The rate of Lisa is 1/13 order per hour.

Bill, a new clerk, needs 16 hours to do the same job.

The rate of Bill is 1/16 order per hour.

Let x represent the number of hours that Bill and Lisa need to go the same job together, x>0.

1/x order per hour is the together rate of Bill and Lisa.

1/13 + 1/16 = 1/x

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x = 208/29 hr

x = 7 hr 5*60/29 min

x = 7 hr 10 min

It will take them 7 hours and 10 minutes to fill the order together.