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Estonia algebra

Problem

Juku thought of a 3-digit number that, when reversing the order of the digits, stays the same 3-digit number. Juku noticed that when adding 2016 to that number, the 4-digit number that arises is again the same 4-digit number when reading the digits from right to left. What number did Juku think of?
Solution
Let the number be and let the number we get by adding be . Clearly can only be or .

If then by the ones digit the only possibility is , and we have a carry from the ones to the tens digit. By the tens digit then or . The second option is impossible, since by the hundreds digit we can only have , if there is no carry from the tens to the hundreds digit, and , if there is a carry from the tens to the hundreds digit. Thus . Then there is no carry to the hundreds digit, hence and .

If then in adding the hundreds digits we must have a carry to the thousands digit which is possible only when . But by the ones digit we should have . The contradiction shows that this case is not possible.
Final answer
646

Techniques

Integers