If the third digit of an acceptable product code is not 0, which one of the following must be true?

Emil-Kunkin on December 14 at 12:05AM

Let's see what we know if the third digit is not zero. We know that zero cannot be the first two digits, and we know it can never be last, since the final digit is greater than the third digit. This means that zero can only ever go third or fourth. Since it cannot go third, it must go fourth.