During January 1 through December 31, 2008, epidemiologists conducted a prevalence survey of type 2 diabetes; 500,000 cases were detected in a population of 10,000,000 persons. It was known that the incidence of diabetes in this population was 10 per 1,000. Estimate the percentage of the prevalent cases that were newly identified during the year.

The answer is 20%. I don't know how to get that answer.

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