Teacher Ying took 720 yuan to the clothing market to purchase. If they all buy T-shirts, they can buy 12 pieces; if they all buy shirts, they can buy 18 pieces. How much does a T-shirt cost compared to a shirt?
应老师带了720元钱去服装市场采购．如果都买T恤，可以买12件，如果都买衬衫，可以买18件．一件T恤比一件衬衫贵多少元？
<gadget id="calculator">720 / 12</gadget>
<output>60</output>

<gadget id="calculator">720 / 18</gadget>
<output>40</output>

<gadget id="calculator">60 - 40</gadget>
<output>20</output>

<result>20</result>