If the latter term of 4:5 becomes 15, what should be the former term of the ratio to keep the ratio unchanged?
如果4：5的后项变成15，要使比值不变，比的前项应该是多少
<gadget id="calculator">15 / 5</gadget>
<output>3</output>

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

<result>12</result>