Ding Ding bought a musical greeting card for 12 yuan, and bought 10 ordinary greeting cards for 30 yuan. How much is a musical greeting card more expensive than an ordinary greeting card?
丁丁买了一张音乐贺卡，用了12元，买了10张普通贺卡用了30元．一张音乐贺卡比一张普通贺卡贵多少元？
<gadget id="calculator">30 / 10</gadget>
<output>3</output>

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

<result>9</result>