Usually all the books in the student bookstore are sold at the fixed price (3/4). This weekend there will be a promotional activity, and the bookstore will give an additional 20% discount on the basis of the original price reduction. It is known that the bookstore is priced with an 80% increase on the purchase price. Xiaohua bought a book and spent 8.1 yuan. How much is the purchase price of this book?
学生书城通常所有图书都以定价的(3/4)出售，这个周末搞促销活动，书店在原来降价的基础上再打八折．已知书城都是在进价的基础上加价80%定价，小华买了一本书花了8.1元，这本书进价多少元？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 + (4/5)</gadget>
<output>9/5 = around 1.8</output>

<gadget id="calculator">8.1 / (4/5) / (3/4) / (9/5)</gadget>
<output>7.5</output>

<result>7.5</result>