3: The former term of 5 is multiplied by 4, to keep the ratio unchanged, how much should be added to the latter term.
3：5的前项乘4，要使比值不变，后项应加上多少．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<result>15</result>