Aunt Wang bought 3 barrels of instant noodles, each barrel of a yuan, paid a total of 20 yuan, how much should I get back.
王阿姨买了3桶方便面，每桶a元，共付了20元，应该找回多少元．
<gadget id="calculator">20 - 3</gadget>
<output>17</output>

<result>17</result>