The two outer terms of the ratio are 0.5 and x, and the two inner terms are 0.8 and 15.
比例的两个外项分别为0.5与x，两个内项分别是0.8和15．
<gadget id="calculator">15 * 0.8</gadget>
<output>12</output>

<gadget id="calculator">12 / 0.5</gadget>
<output>24</output>

<result>24</result>