A ratio is 4:5, if the ratio of this ratio remains unchanged, the former item is increased by 4, and the latter item should be increased by how much.
一个比是4：5，如果这个比的比值不变，前项增加4，后项应增加多少．
<gadget id="calculator">5 * 2</gadget>
<output>10</output>

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

<result>5</result>