The store bought a batch of sandals at 6.5 yuan per pair and sold them at 8.7 yuan per pair. When 25% of the sandals were sold, the store not only recovered the money paid for the purchase of these sandals, but also made a profit of 20 yuan. How many pairs of sandals are there in total?
商店以每双6.5元购进一批凉鞋，售价为每双8.7元，当卖剩下25%时，不仅收回了购进这批凉鞋所付出的款项，而且获利20元．这批凉鞋共有多少双？
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">8.7 * (3/4)</gadget>
<output>6.525</output>

<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>