Aunt Li brought 500 yuan to buy fish, bought 25 kilograms, and left 50 yuan. What is the unit price of fish?
李阿姨带了500元去买鱼，买了25千克，还剩下50元．鱼的单价是多少？
<gadget id="calculator">500 - 50</gadget>
<output>450</output>

<gadget id="calculator">450 / 25</gadget>
<output>18</output>

<result>18</result>