The reservoir manager wants to estimate how many fish there are in the reservoir. He fishes 100 fish as samples, marks all of them and puts them back into the reservoir. A month later, he caught 500 fish and found that only 4 of them had been marked, so he estimated how many fish there were in the reservoir.
水库管理员想估算一下水库里共有多少条鱼，他先捞了100条作为样本，全部做上记号后放回水库．一个月后，他捕捞了500条鱼，发现其中只有4条鱼做过记号，由此他估测水库中大约有多少条鱼．
<gadget id="calculator">4 / 500</gadget>
<output>1/125 = around 0.008</output>

<gadget id="calculator">100 / (1/125)</gadget>
<output>12_500</output>

<result>12_500</result>