Yahoo Inc. unveiled a plan to spin off tax-free its nearly $40 billion of holdings in Alibaba Group Holding Ltd., a move that should give Chief Executive Marissa Mayer more time with shareholders despite continued declines in the Internet portal’s core advertising business.

Investors have been eager to hear Yahoo’s plans to extract value from its Asian assets, which represent the vast majority of its $47 billion market value, while avoiding a tax bill of billions of dollars.

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