The store bought a pair of sandals for 6.50 yuan and sold a batch of sandals for 8.70 yuan. When the surplus (1/4) is sold, not only the cost of purchasing these shoes is recovered, but also 20 yuan is obtained. How many pairs of this batch of shoes are there?
商店以每双6.50元购进，8.70元售出一批凉鞋．当卖剩(1/4)时，不仅收回了购这批鞋的成本，且获20元，这批鞋有多少双？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">8.7 * (1/4)</gadget>
<output>2.175</output>

<gadget id="calculator">8.7 - 2.175 - 6.5</gadget>
<output>0.025</output>

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

<result>800</result>