Wang Jun buys a pen for 15 yuan and a notebook for 2 yuan, pays the salesperson 20 yuan, and asks how much to get back. Which of the following is incorrect
王军买一支15元的钢笔和一本2元的笔记本，付给售货员20元钱，求应找回多少元．下面列式不正确的是多少
<gadget id="calculator">15 - 2</gadget>
<output>13</output>

<gadget id="calculator">20 - 13</gadget>
<output>7</output>

<result>7</result>