A store buys a batch of badminton rackets, the purchase price is 30 yuan per pair, and the retail price is 40 yuan per pair. When there are 100 pairs left, the money used to buy these rackets has been recovered, and the profit is 100 yuan. How many pairs are there in this batch of badminton rackets?
某商店购进一批羽毛球拍，进价每对30元，零售价每对40元，当还剩下100对时，已收回购进这批球拍所用的钱之外，还盈利100元．这批羽毛球拍共有多少对．
<gadget id="calculator">100 * 40</gadget>
<output>4_000</output>

<gadget id="calculator">4_000 + 100</gadget>
<output>4_100</output>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<gadget id="calculator">4_100 / 10</gadget>
<output>410</output>

<result>410</result>