The ratio is 4:15, when adding 8 to the former item, how much must be added to the latter item, and the ratio remains unchanged.
比4：15的前项加上8，后项必须加上多少，比值不变．
<gadget id="calculator">15 * 3</gadget>
<output>45</output>

<gadget id="calculator">45 - 15</gadget>
<output>30</output>

<result>30</result>