Mom bought 4 kilograms of bananas and 3.5 kilograms of apples, which cost a total of 51.2 yuan. It is known that bananas are 7.2 yuan per kilogram, how much is an apple per kilogram?
妈妈买了4千克香蕉和3.5千克苹果，一共花了51.2元．已知香蕉每千克7.2元，苹果每千克多少元？
<gadget id="calculator">4 * 7.2</gadget>
<output>28.8</output>

<gadget id="calculator">51.2 - 28.8</gadget>
<output>22.4</output>

<gadget id="calculator">22.4 / 3.5</gadget>
<output>6.4</output>

<result>6.4</result>