In the ratio 4:5=2:2.5, if 4 is added to the former term of the first ratio, how much should be added to the former term of the second ratio to make this ratio still valid.
在比例4：5=2：2.5中，如果第一个比的前项加上4，要使这个比例仍然成立，第二个比的前项应加上多少．
<gadget id="calculator">4 + 4</gadget>
<output>8</output>

<gadget id="calculator">2.5 / 2</gadget>
<output>1.25</output>

<gadget id="calculator">8 * 1.25</gadget>
<output>10</output>

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

<result>5</result>