Grandma Zhang bought 4.5 kilograms of cucumbers in the market, paid 20 yuan, and got 2 yuan back. How much per kilogram of cucumber?
张奶奶在市场里买了4.5千克黄瓜，付了20元，找回2元．每千克黄瓜多少元？
<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<gadget id="calculator">18 / 4.5</gadget>
<output>4</output>

<result>4</result>