The teacher is going to distribute exercise books to 20 students. If there are still some exercise books for each of the two, and three copies for each person is not enough, the leftovers are as much as the insufficient ones. How many exercise books were there?
老师准备给20同学发练习本，如果每人两本还剩一些，每人3本又不够发，剩下的和不够的同样多．原来有多少本练习本？
<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<gadget id="calculator">20 * 3</gadget>
<output>60</output>

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

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">40 + 10</gadget>
<output>50</output>

<result>50</result>