Xiao Ming brought some money to buy hard-faced banknotes, and after buying two copies, there was still 1.80 yuan left; if he bought 4 copies of the same hard-faced banknote, the difference was 2.40 yuan, so how much did Xiao Ming bring?
小明带一些钱去买硬面钞，买两本后还剩1.80元；如果买同样的硬面钞4本则差2.40元，那么小明带了多少钱？
<gadget id="calculator">1.8 + 2.4</gadget>
<output>4.2</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">4.2 / 2</gadget>
<output>2.1</output>

<gadget id="calculator">2.1 * 2</gadget>
<output>4.2</output>

<gadget id="calculator">4.2 + 1.8</gadget>
<output>6</output>

<result>6</result>