A bamboo shoot, from germination to growth, if it doubles in height every day, it will grow to 40 decimeters in 10 days. So how many days does it take to grow to 2.5 decimeters?
一根竹笋，从发芽到长大，如果每天长高一倍，经过10天长到40分米．那么长到2.5分米时，需要经过多少天．
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