设这批零件共有x只，

1

7

x+（

1

7

x+24）+（

1

7

x+24）×

1

2

=x
         

1

7

x+

1

7

x+24+

1

14

x+12=x
              x-

1

7

x-

1

7

x-

1

14

x=36
                       

9

14

x=36
                         x=56
答：这批零件共有56只．