22.5 minutes
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Let's assume Mary's time to set the tables by herself is M minutes.
Bob's work rate is 1/18 of the job per minute.
When Bob and Mary work together, their combined work rate is 1/10 of the job per minute.
From this, we set up the equation for their combined work rate:
To solve for M, follow these steps:
Mary would take 22.5 minutes to set the tables by herself.