Xiaoli has 36 extracurricular books, and Xiaohong has 42 extracurricular books. After Xiaohong gave Xiaoli a few books, they both had the same number of extracurricular books.
小丽有36本课外书，小红有42本课外书．小红给小丽多少本后，两人的课外书一样多．
<gadget id="calculator">36 + 42</gadget>
<output>78</output>

<gadget id="calculator">78 / 2</gadget>
<output>39</output>

<gadget id="calculator">42 - 39</gadget>
<output>3</output>

<result>3</result>