x, 10, 4, 0.5 can form a proportion, what is the maximum x.
x、10、4、0.5能组成比例，x最大是多少．
<gadget id="calculator">4 / 0.5</gadget>
<output>8</output>

<gadget id="calculator">8 * 10</gadget>
<output>80</output>

<result>80</result>