Xiaogang reads a storybook. He originally planned to finish it in 10 days, but he actually read 5 more pages a day. He finished it 2 days earlier. How many books did he plan to read every day?
小刚看一本故事书，原计划10天看完，实际每天多看5页，提前2天看完，原计划每天看多少本书？
<gadget id="calculator">10 * 5</gadget>
<output>50</output>

<gadget id="calculator">2 * 5</gadget>
<output>10</output>

<gadget id="calculator">50 - 10</gadget>
<output>40</output>

<gadget id="calculator">2 - 10</gadget>
<output>-8</output>

<gadget id="calculator">10 + (-8)</gadget>
<output>2</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<result>20</result>