In a ratio, the two internal terms are reciprocals of each other, one of the external terms is (1/2), and the other external term is how much.
在一个比例中，两个内项互为倒数，其中一个外项是(1/2)，另一个外项是多少．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / (1/2)</gadget>
<output>2</output>

<result>2</result>