math challenge

Week of June 17 to 23, 1996

Kewl CDs

Kevin has 25 kewl CDs, including some from up-and-coming garage bands. Nine of the CDs play for less than 20 minutes. Twelve play for more than 30 minutes. Four play for more than 40 minutes. How many of Kevin's CDs play between 30 and 40 minutes?

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