Xiao Ming bought 5 kg of apples for 16.5 yuan, and Xiao Hong bought 7 kg of apples for 21.7 yuan. Who bought more expensive? How much is it per kilogram?
小明买5千克苹果用去16.5元，小红买7千克苹果用去21.7元，谁买的贵？每千克贵多少元？
<gadget id="calculator">16.5 / 5</gadget>
<output>3.3</output>

<gadget id="calculator">21.7 / 7</gadget>
<output>3.1</output>

<gadget id="calculator">3.3 - 3.1</gadget>
<output>0.2</output>

<result>0.2</result>