A water glass is 5.8 yuan, and a schoolbag is 2.9 yuan more than a water glass. Xiao Ming bought a water glass, a schoolbag and a pencil case and spent a total of 20 yuan. How much is a pencil case?
一个水杯5.8元，一个书包比一个水杯多2.9元，小明买了一个水杯、一个书包和一个笔盒共花了20元，一个笔盒多少元？
<gadget id="calculator">5.8 + 2.9</gadget>
<output>8.7</output>

<gadget id="calculator">20 - 5.8 - 8.7</gadget>
<output>5.5</output>

<result>5.5</result>