A shoe store bought a batch of cloth shoes at a price of 6.5 yuan per pair. Sold at a price of 8.7 yuan per pair. When 75% of these cloth shoes are sold, not only the original cost is recovered, but also a profit of 20 yuan is made. How many pairs of these shoes did you buy?
一个鞋店以每双6.5元的价钱购进一批布鞋．以每双8.7元的价钱售出．当卖出这批布鞋的75%时，不仅收回原来的成本，而且还盈利20元．购进这批鞋共有多少双？
<gadget id="calculator">6.525 - 6.5</gadget>
<output>0.025</output>

<gadget id="calculator">20 / 0.025</gadget>
<output>800</output>

<result>800</result>