Answer :
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:
- (1/18) + (1/M) = 1/10
To solve for M, follow these steps:
- (M + 18) / (18M) = 1/10
- 10(M + 18) = 18M
- 10M + 180 = 18M
- 180 = 8M
- M = 180 / 8 = 22.5
Mary would take 22.5 minutes to set the tables by herself.