Subtract 8 from the former term of 12:15, to keep the ratio constant, what should be the latter term.
把12：15的前项减去8，要使比值不变，后项应该多少．
<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<gadget id="calculator">12 / 4</gadget>
<output>3</output>

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

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<result>10</result>