Decrease the latter term of 12:20 by 15, how much should the former term be reduced to keep the ratio unchanged.
把12：20的后项减小15，要使比值不变，前项应该减小多少．
<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

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

<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<result>9</result>