Employers ask this question to gauge how willing a candidate is to “go the extra mile” and assume additional responsibilities when necessary. Here are two examples of how to answer such a question:

I would call her first, and see if she was just held up in traffic, or was dealing with an emergency somewhere. Then I would call the manager to see if there was someone who could come to relieve me, and I would stay until my replacement came in.

I wouldn’t leave the floor understaffed. I would check with the manager first, and make every effort to make the necessary arrangements to stay until coverage was found. I might need to close the shop for fifteen minutes to pick up my kids from school if I couldn’t find a mother’s helper at the last minute, but I would come back immediately and stay until a replacement came.