Xiao Ming reads a story book and reads 18 pages a day. He only reads half of the book in 7 days. From then on, he reads 3 more pages every day. How many days did it take Xiao Ming to finish the book?
小明看一本故事书，每天看18页，7天只看了这本书的一半，从此后他每天多看3页，小明看完这本书共用了多少天？
<gadget id="calculator">18 + 3</gadget>
<output>21</output>

<gadget id="calculator">7 / 21</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">18 * (1/3)</gadget>
<output>6</output>

<gadget id="calculator">6 + 7</gadget>
<output>13</output>

<result>13</result>