The supermarket bought back 5 boxes of apples. Each box of apples weighed 20 kilograms. It cost 400 yuan to buy these apples. How much is the price of this kind of apples per kilogram?
超市购回了5箱苹果，每箱苹果重20千克，买回这些苹果花了400元，这种苹果每千克售价多少元？
<gadget id="calculator">5 * 20</gadget>
<output>100</output>

<gadget id="calculator">400 / 100</gadget>
<output>4</output>

<result>4</result>