Li Xiaogang bought a pencil case and used half of the money he brought, and bought a notebook for another 4 yuan. At this time, there was 21 yuan left. How much did Li Xiaogang bring?
李小刚买了一个铅笔盒用去所带钱的一半，买一本笔记本又用去4元，这时还剩21元，李小刚原来带了多少钱？
<gadget id="calculator">21 + 4</gadget>
<output>25</output>

<gadget id="calculator">25 * 2</gadget>
<output>50</output>

<result>50</result>