Add 4 to the latter term of 2:4, how much should the former term be multiplied to keep the ratio constant
在2﹕4的后项加上4，要使比值不变前项应乘以多少
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<result>2</result>