Wang Hong and Zhang Hua bought the same exercise books. Wang Hong bought 6 copies and spent 3.6 yuan; Zhang Hua spent 7.8 yuan. How many more copies did Zhang Hua buy than Wang Hong?
王洪与张华买同样的练习本，王洪买了6本，用去了3.6元；张华用去了7.8元．张华比王洪多买了几本？
<gadget id="calculator">3.6 / 6</gadget>
<output>0.6</output>

<gadget id="calculator">7.8 / 0.6</gadget>
<output>13</output>

<gadget id="calculator">13 - 6</gadget>
<output>7</output>

<result>7</result>