Mr. Wang brought some money to buy a reference book as a prize. With the money, he could buy 8 volumes 1 or 10 volumes 2. Now he has bought a volume 2. If the rest of the money is purchased as a set, how many sets can he buy? reference book.
王老师带一些钱去买一种工具书作奖品，这些钱可买8本上册或10本下册，现己买了一本下册书，余下的钱若配套买，还可买多少套这样的工具书．
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(1/8) + (1/10)</gadget>
<output>9/40 = around 0.225</output>

<gadget id="calculator">(9/10) / (9/40)</gadget>
<output>4</output>

<result>4</result>