Aunt Zhang bought 3.4 kilograms of beans in the supermarket, paid 20 yuan, and got back 8.1 yuan. How much is it per kilogram of beans?
张阿姨在超市买了3.4千克豆角，付了20元，找回8.1元，每千克豆角多少元？
<gadget id="calculator">20 - 8.1</gadget>
<output>11.9</output>

<gadget id="calculator">11.9 / 3.4</gadget>
<output>3.5</output>

<result>3.5</result>