Teacher Li reads a book and reads 105 pages every day. How many pages should he start reading on the fifth day?
李老师读一本书，每天读105页，第五天应从第多少页开始读起．
<gadget id="calculator">105 * 4</gadget>
<output>420</output>

<gadget id="calculator">420 + 1</gadget>
<output>421</output>

<result>421</result>