If the former term of (3/7) is increased by 12, how much should the latter term be increased to keep the ratio unchanged.
(3/7)的前项增加12，后项应该增加多少，才能保持比值不变．
<gadget id="calculator">3 + 12</gadget>
<output>15</output>

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<gadget id="calculator">5 * 7</gadget>
<output>35</output>

<gadget id="calculator">35 - 7</gadget>
<output>28</output>

<result>28</result>