Teacher Li bought clothes for the students. 13 T-shirts cost 728 yuan, and 15 pairs of trousers cost 525 yuan. How much more expensive is a T-shirt than a pair of trousers?
李老师给同学们买服装，13件T恤用了728元，15条裤子用了525元，一件T恤比一条裤子贵多好元？
<gadget id="calculator">56 - 35</gadget>
<output>21</output>

<result>21</result>