Maths Olympiad Prep

Track / Stage 6 / 6 of 400 #1006 of 1964

Problem 1006

National olympiad, first round
Number theory Difficulty 6.0 Prove it

7. Let a, b, c be three natural numbers. On the board, the three products ab,ac,bcab, ac, bc were written, and in each of them, all digits except the last two were erased. Could it happen that the result was three consecutive two-digit numbers? (N. Agakhanov)

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

Answer: It could not. Solution: Suppose the opposite: the products ab,bcab, bc, and caca end with the two-digit numbers n,n+1n, n+1, and n+2n+2, respectively. Among these three consecutive numbers, there must be an odd number, meaning the product of some two of the numbers a,ba, b, and cc is odd. This implies that at least two of the numbers a,ba, b, and cc are odd. But then the third number is even, otherwise all three products ab,bcab, bc, and caca would be odd, which is impossible.

Thus, among the products, one is odd and two are even, meaning the number nn is even. Then the number aa is even, and the numbers bb and cc are odd. Now, if aa is divisible by 4, then both numbers nn and n+2n+2 must be divisible by 4. If aa is not divisible by 4, then the numbers nn and n+2n+2 are also not divisible by 4. However, among the two consecutive numbers nn and n+2n+2, one must be divisible by 4, and the other is not. Contradiction.

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