The ratio 4:3=12:9 subtracts 1 from the inner term 3, how much should the outer term 9 be to make the ratio valid.
比例4：3=12：9的内项3减去1，要使比例成立，外项9应该多少．
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

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

<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">9 - 6</gadget>
<output>3</output>

<result>3</result>