The survival rate of a tree is 80%, how many of the 200 trees survived and died.
一种树成活率是80%，成活200棵，死了多少棵．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">200 / (4/5)</gadget>
<output>250</output>

<gadget id="calculator">250 - 200</gadget>
<output>50</output>

<result>50</result>