A small fish doubles its mass in one day, and grows to 20 grams in 20 days. How many days will it take to grow to 5 grams?
一条小鱼一天质量增长一倍，20天长到了20克，当长到5克时需要多少天．
<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<result>18</result>