Zhou Yan, Liu Min, and Zhang Xin each had some stickers. Zhou Yan gave Liu Min 13 and Liu Min gave Zhang Xin 15. The three had the same number of stickers. It turned out that Liu Min had more stickers than Zhang Xin.
周燕、刘敏、张欣各有一些贴画，周燕给刘敏13张，刘敏给张欣15张后，三人的贴画张数就一样多，原来刘敏比张欣多多少张．
<gadget id="calculator">15 - 13</gadget>
<output>2</output>

<gadget id="calculator">15 + 2</gadget>
<output>17</output>

<result>17</result>