7. Let a, b, c be three natural numbers. On the board, the three products 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)
Problem 1006
Official solution
Answer: It could not. Solution: Suppose the opposite: the products , and end with the two-digit numbers , and , respectively. Among these three consecutive numbers, there must be an odd number, meaning the product of some two of the numbers , and is odd. This implies that at least two of the numbers , and are odd. But then the third number is even, otherwise all three products , and would be odd, which is impossible.
Thus, among the products, one is odd and two are even, meaning the number is even. Then the number is even, and the numbers and are odd. Now, if is divisible by 4, then both numbers and must be divisible by 4. If is not divisible by 4, then the numbers and are also not divisible by 4. However, among the two consecutive numbers and , one must be divisible by 4, and the other is not. Contradiction.