Add 15 to the former item of 3:5, if the ratio remains unchanged, how much should be added to the latter item.
3：5的前项加上15，若比值不变，后项应加多少．
<gadget id="calculator">3 + 15</gadget>
<output>18</output>

<gadget id="calculator">18 / 3</gadget>
<output>6</output>

<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<gadget id="calculator">30 - 5</gadget>
<output>25</output>

<result>25</result>