If a bamboo shoot grows from germination to growth, if it doubles every day and grows to 40 decimeters in 10 days, then when it grows to 2.5 decimeters, how many days does it take for a bamboo shoot to grow from germination to growth?
一根竹笋从发芽到长大，如果每天长一倍，经过10天长到40分米，那么长到2.5分米时，一根竹笋从发芽到长经过了多少天．
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