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Matthias and Klara live in a high-rise building. Klara lives 12 floors above Matthias.
One day, Matthias climbs the staircase to visit Klara.
When the amount of floors he has traveled equals the number of remaining floors, he is on the 8th floor.
On which floor does Klara live ?
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Mental solution
When Matthias was on 8-th floor, the number of the remaining floors to climb was 12/2 = 6, according to the condition.
Hence, Klara lives on the 8+6 = 14 (fourteenth) floor.
ANSWER. Klara lives on the 14-th floor.
CHECK. Then Matthias lives at the 14-12 = 2 (second) floors.
14-8 = 6 floors to climb, and 8-2 = 6 floors to climb,
which is consistent with the problem's description.
Solved mentally.
This simple primitive problem does not require any more complicated technique.
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This problem is really simple and primitive (at the level of the 3-rd grade),
but it teaches to read the problem attentively to understand its meaning in full.
It also develops the student's common sense and teaches a student to think
independently and to organize his or her thoughts to express the solution
in simple and clear manner. So, although the problem is simple, it has a significant educational value.
I think that if a student will solve, let say, 10 (ten) Math problems of this kind on his/her own
in his/her 3rd grade, it would create a good mental charge for his/her mind for all the following years.
The problems of this kind are real treasures for the young mind.