Xiao Ming goes to the bookstore to buy books. If he buys 2 books, there will be 1.8 yuan left; if he buys the same 4 books, the difference will be 2.4 yuan. How much money does Xiao Ming have on him?
小明去书店买书，如果买2本，则还剩1.8元；如果买同样的4本，则差2.4元．小明身上有多少元钱．
<gadget id="calculator">2.4 + 1.8</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>