A worm grows from a larva to an adult, and its weight doubles every day, and grows to 256 grams in 30 days. How many days does it take for the worm to grow to 2 grams?
一条小虫由幼虫长到成虫，每天的重量增加一倍，30天长到256克，那么小虫长到2克时需要多少天．
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