Xiao Ming uses some money to buy homework books. If he buys 6 books, there will be 0.90 yuan left, and if he buys 9 books, there will be a difference of 0.60 yuan. How much money does Xiao Ming originally have?
小明用一些钱买作业本，如果买6本就剩余0.90元，如果买9本就还差0.60元，小明原来有多少元钱．
<gadget id="calculator">0.9 + 0.6</gadget>
<output>1.5</output>

<gadget id="calculator">9 - 6</gadget>
<output>3</output>

<gadget id="calculator">1.5 / 3</gadget>
<output>0.5</output>

<gadget id="calculator">6 * 0.5</gadget>
<output>3</output>

<gadget id="calculator">3 + 0.9</gadget>
<output>3.9</output>

<result>3.9</result>