Add 2 to the former term of 4:15, to keep the ratio unchanged, how much to add to the latter term.
把4：15的前项加上2，要使比值不变，后项加上多少．
<gadget id="calculator">15 * 1.5</gadget>
<output>22.5</output>

<gadget id="calculator">22.5 - 15</gadget>
<output>7.5</output>

<result>7.5</result>