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I might be free on some afternoons is the most popular phrase on the web. 

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• are you free on saturday or are you free on saturdays?
• I was on drugs. or I was on some drugs. ?
• i shall be available on phone or I may be available on phone?
• I could be of any help or I could be of any helps?
• I might be free on some afternoons or I might be free in some afternoons?