What is incidence of this gripping disease in 2007?

A fictitious disease, Rome On HBO Fever, is slowly taking over the entire NY medical student community (3,000 students). In 2006, the prevalence of this disease within this community was 10%. In 2007, 270 new cases of Rome On HBO Fever were reported. What is incidence of this gripping disease in 2007?

    1. 5%
    1. 9%
    1. 10%
    1. 15%
    1. 19%

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exp:

Incidence = (# of new cases) / (# at risk). For 2007, there are 270 new cases and 2700 people at risk; therefore, the incidence is 10%. Do not forget to limit the incidence value to only those at risk. The 300 infected from 2006 (10% of 3000) are no longer at risk for the disease–they already have it!