There are 10 pencil cases, 5 of which contain pencils, 4 of which contain pens, and 2 of which contain both pencils and pens. How many empty pencil cases are there?
有10个铅笔盒，其中5个装有铅笔，4个装有钢笔，2个既装有铅笔又有钢笔，空笔盒有多少个．
<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">5 + 2</gadget>
<output>7</output>

<gadget id="calculator">10 - 7</gadget>
<output>3</output>

<result>3</result>