13: Multiply the former term of 5 by 2, how much should be added to the latter term to keep the ratio constant.
13：5的前项乘2，要使比值不变，后项应加多少．
<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 5</gadget>
<output>5</output>

<result>5</result>