Xiaohong goes to the supermarket to buy watermelon, 1.8 yuan per kilogram of watermelon, Xiaohong pays 30 yuan, and the cashier gets back 3.9 yuan. How many kilograms of watermelon did Xiaohong buy?
小红去超市买西瓜，每千克西瓜1.8元，小红付了30元，收银员找回3.9元．小红买了多少千克西瓜？
<gadget id="calculator">30 - 3.9</gadget>
<output>26.1</output>

<gadget id="calculator">26.1 / 1.8</gadget>
<output>14.5</output>

<result>14.5</result>