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A box contains 15 identical reams of paper.
The total mass of the box with 1/5 of all the reams of papers
is 33.6 kg lighter than the box with al the reams of paper.
The mass of the empty box is 2/7 of a ream of paper.
What is the mass of the empty box?
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Usually, it is assumed, that after reading such problem,
the student will solve it MENTALLY in a few lines.
Indeed, compare these two statements
A box contains 15 identical reams of paper.
The total mass of the box with 1/5 of all the reams of papers
is 33.6 kg lighter than the box with al the reams of paper.
From this reading, it should be clear that 12 = 15 - 3 reams of paper weight 33.6 kg,
and hence, one ream of paper weights 33.6/12 = 2.8 kg.
One simple step further, and we answer the question: the mass of the empty box is = 2*0.4 = 0.8 kg.
ANSWER. The mass of the empty box is 0.8 kilograms.
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It is the most detailed solution, which should accompany this problem.
I am 179% sure that the problem is expected to be solved this way, in a few lines and a few words.