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Print51st Ukrainian National Mathematical Olympiad, 3rd Round
Ukraine counting and probability
Problem
There are 10 piles of stones, with , , , , stones respectively. At one step one can pick three piles and add stone to the first pile, stones to the second pile, stones to the third pile, or pick any three piles and take stone out of first pile, stones out of second pile, stones out of third pile, provided that each pile has enough stones. Is it possible after finite number of such operations to get exactly stones in each pile?
Solution
After each operation the total number of stones changes by a number which is divisible by . At the end the total number is which is not divisible by . However, at the starting moment the total number is divisible by , which provides a contradiction.
Final answer
No
Techniques
Invariants / monovariants