The older brother and the younger brother each buy a number of exercise books. If the older brother gives the younger brother 3 copies, they both have the same number of exercise books; How many exercise books did my younger brother buy?
哥哥和弟弟各买若干本练习本，如果哥哥给弟弟3本，两人的练习本数量就同样多；如果弟弟给哥哥1本，哥哥的练习本本数就是弟弟的3倍．弟弟原来买练习本多少本．
<gadget id="calculator">3 + 6 + 1</gadget>
<output>10</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<result>5</result>