A copy of review materials is 6.4 yuan after a 20% discount. What is the original price of this book?
一本复习资料，打八折后是6.4元，这本书原价是多少元．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">6.4 / (4/5)</gadget>
<output>8</output>

<result>8</result>