Uncle Liu brought 640 yuan to buy feed and bought 20 bags of feed. With 20 yuan left, what is the price of each bag of feed?
刘叔叔带了640元买饲料，买了20袋饲料．剩下20元，每袋饲料的价钱是多少？
<gadget id="calculator">640 - 20</gadget>
<output>620</output>

<gadget id="calculator">620 / 20</gadget>
<output>31</output>

<result>31</result>