There are 51 books on the first shelf, and the number of books on the second shelf is three times that of the first shelf. How much is the difference between the books on the two shelves?
第一层书架上有51本书，第二层书架上的书是第一层书架的3倍，两层书架上的书相差多少本？
<gadget id="calculator">51 * 3</gadget>
<output>153</output>

<gadget id="calculator">153 - 51</gadget>
<output>102</output>

<result>102</result>