If you add 4 to the former term of 2:5, how much should be added to the latter term to keep the ratio of fractions unchanged?
如果把2：5的前项加上4，要使分数比值不变，后项应加上多少
<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<result>10</result>