Xiaohua bought 3.5 kilograms of apples for Granny Li, paid 15 yuan to the salesperson's aunt, and got back 2.4 yuan. How much is an apple per kilogram?
小华帮李婆婆买了3.5千克的苹果，付给售货员阿姨15元，找回2.4元．苹果每千克多少元？
<gadget id="calculator">15 - 2.4</gadget>
<output>12.6</output>

<gadget id="calculator">12.6 / 3.5</gadget>
<output>3.6</output>

<result>3.6</result>