Aunt Li bought 3.5kg of apples and paid the salesperson 20 yuan, and the salesperson retrieved 5.3 yuan for Aunt Li. How much is it per kilogram of apples?
李阿姨买了3.5kg苹果，付给售货员20元，售货员找回李阿姨5.3元，每千克苹果多少元？
<gadget id="calculator">20 - 5.3</gadget>
<output>14.7</output>

<gadget id="calculator">14.7 / 3.5</gadget>
<output>4.2</output>

<result>4.2</result>