A and B bought 20 exercise books of the same specification and each paid half of the price. A wants 4 more copies than B, so A refunds B 4 yuan. How much is each copy of this exercise book?
甲、乙两人买了20本相同规格的练习册，各付一半钱．甲比乙多要4本，所以甲又退给乙4元钱．这种练习册每本多少元．
<gadget id="calculator">8 / 4</gadget>
<output>2</output>

<result>2</result>