Mao Mao and Xiao Bing bought the same exercise book in the same store. Mao Mao bought 9 books, and Xiao Bing bought 6 books. Mao Mao paid 3 yuan and 9 cents more than Xiao Bing. What is the price of each exercise book.
毛毛和小兵在同一个商店买同一种练习本，毛毛买了9本，小兵买了6本，毛毛比小兵多付了3元9角钱．每本练习本的价格是多少．
<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">3 + (9/10)</gadget>
<output>39/10 = around 3.9</output>

<gadget id="calculator">9 - 6</gadget>
<output>3</output>

<gadget id="calculator">(39/10) / 3</gadget>
<output>13/10 = around 1.3</output>

<result>13/10 = around 1.3</result>