Students from three classes in the sixth grade helped the library repair books. The first class repaired 56 books, and the second class repaired the first class (7/8). The third shift repaired less than the second shift (3/7). How many copies did the third class repair?
六年级三个班学生帮助图书室修补图书．一班修补了56本，二班修补的是一班的(7/8)．三班修补的比二班少(3/7)．三班修补了多少本？
<gadget id="calculator">7 / 8</gadget>
<output>7/8 = around 0.875</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">1 - (3/7)</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">56 * (7/8) * (4/7)</gadget>
<output>28</output>

<result>28</result>