Xiaoqiang made a set of exercises. He originally planned to finish it in 30 days. After 4 days as planned, he had 2 more questions per day than originally planned. After doing this for 10 days, he did 2 more questions every day. The result was exactly 6 days ahead of schedule. After completing all the exercises, how many exercises are there in this book?
小强做一本习题集，原计划30天完成，按计划做了4天后，他每天比原计划多2道题，这样做了10天后，他每天又多做2道题目，结果恰好提前6天做完了全部习题，这本书中共有多少道习题？
<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">30 - 4 - 10 - 6</gadget>
<output>10</output>

<gadget id="calculator">2 + 2</gadget>
<output>4</output>

<gadget id="calculator">10 * 4</gadget>
<output>40</output>

<gadget id="calculator">20 + 40</gadget>
<output>60</output>

<gadget id="calculator">60 / 6</gadget>
<output>10</output>

<gadget id="calculator">10 * 30</gadget>
<output>300</output>

<result>300</result>