Olympiad Maths Prep

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Problem 683

AIME late
Number theory Difficulty 5.3 Find the answer

10.9 A two-digit number divided by its reverse yields a quotient that is exactly equal to the remainder. Find this two-digit number.
(China Beijing Junior High School Grade 2 Mathematics Competition, 1991)

Official solution

[Solution] Let the two-digit number be a=10x+ya=10 x+y, then its reverse number is 10y+x10 y+x, where 1x9,0y9,x,yZ1 \leqslant x \leqslant 9, \quad 0 \leqslant y \leqslant 9, \quad x, y \in Z.
From the problem, we have
10x+y=(10y+x)q+q,qN. 10 x+y=(10 y+x) q+q, \quad q \in N .

That is
(10q)x(10q1)y=q (10-q) x-(10 q-1) y=q \text {. }
(1) When q=1q=1, (1) becomes 9x9y=19 x-9 y=1.
This is impossible;
(2) When q=2q=2, (1) becomes 8x19y=28 x-19 y=2.
Thus, yy is even.
When y=2y=2, x=5x=5, at this time a=52a=52.
When y=4,8y=4,8, the right side of the equation cannot be divided by 4, while the left side can be divided by 4, so there is no solution.
When y=6y=6, it is easy to verify that there is no solution.
Therefore, when q=2q=2, a=52a=52.
(3) When q=3q=3, (1) becomes
7x29y=3. 7 x-29 y=3 .

When y2y \leqslant 2, xx is not an integer.
When y3y \geqslant 3,
7x=29y+390,x>10. 7 x=29 y+3 \geqslant 90, \quad x>10 .

So, when q=3q=3, there is no solution.
(4) When q=4q=4, (1) becomes
6x39y=4 6 x-39 y=4 \text {. }

The left side of this equation can be divided by 3, but the right side cannot be divided by 3, so there is no integer solution.
(5) When q5q \geqslant 5, we have
5x(10q)x=(10q1)y+q49y+q. \begin{aligned} 5 x & \geqslant(10-q) x \\ & =(10 q-1) y+q \\ & \geqslant 49 y+q . \end{aligned}

If y1y \geqslant 1, then 5x49y+q545 x \geqslant 49 y+q \geqslant 54, thus x>10x>10, which is impossible.
If y=0y=0, then from (1) we get
q=(10q)x10q. q=(10-q) x \geqslant 10-q .

This gives x=1,q=5x=1, q=5 or x=4,q=8x=4, q=8, which is also impossible.
In summary, the only solution to this problem is a=52a=52.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.