The two outer terms of a ratio are reciprocals of each other, one inner term is 2.5, and the other inner term is what.
一个比例的两外项互为倒数，一个內项是2.5，另一个內项是多少．
<gadget id="calculator">1 / 2.5</gadget>
<output>0.4</output>

<result>0.4</result>