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The number of people visiting a zoo during weekends is given below. Determine the value of "\(f\)" if the mean of the following data is 30.
| Age | \(0 - 10\) | \(10 - 20\) | \(20 - 30\) | \(30 - 40\) | \(40 - 50\) | \(50 - 60\) | \(60 - 70\) |
| Number of people | 9 | 9 | \(f\) | 4 | 2 | 9 | 5 |
The value of the variable is .