Xiaoqiang made a set of exercises, which was originally planned to be completed in 30 days. After studying for 8 days as planned, he did 6 more questions every day than originally planned. As a result, he finished all the exercises 4 days ahead of schedule. How many exercises are there in this book?
小强做一本习题集，原计划30天完成．按计划学习8天后，他每天比原计划多做6题，结果提前4天完了全部习题，这本书共有多少道习题．
<gadget id="calculator">30 - 8 - 4</gadget>
<output>18</output>

<gadget id="calculator">18 / 4</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">6 * (9/2) * 30</gadget>
<output>810</output>

<result>810</result>