Question 1190207
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            You can solve this problem,  as the other tutor has shown in his post.


            Or,  alternatively,  you can solve it mentally,  without using equations.



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The difference between the first case items and the second case items is 2/3 of the box of ball bearings, which weights 21 - 9 = 12 pounds.


Hence, 1/3 of the box of ball bearing weights 6 pounds.


It means that the full box of ball bearings weights 6*3 = 18 pounds (with no box itself),


and the box itself weights 21-18 = 3 pounds.   <U>ANSWER</U>
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Solved.