There are 40 questions in the mathematics competition, 2.5 points are awarded for one correct one, and 1.5 points are deducted for one wrong one. As a result, Wang Gang scored 60 points. How many questions did he get wrong?
数学竞赛有40道题，算对一道给2.5分，算错一道倒扣1.5分，结果王刚得60分，他错了几道题？
<gadget id="calculator">2.5 * 40</gadget>
<output>100</output>

<gadget id="calculator">100 - 60</gadget>
<output>40</output>

<gadget id="calculator">2.5 + 1.5</gadget>
<output>4</output>

<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<result>10</result>