Why all do odd numbers must end in one and five digits?

Odd numbers are defined as integers that are not divisible by 2. When considering the decimal system, which uses base 10, the digits range from 0 to 9. The digits that denote odd numbers are 1, 3, 5, 7, and 9; therefore, any odd number will end with one of these digits. In the context of your question, it seems there might be a misunderstanding, as odd numbers can end in 1, 3, 5, 7, or 9, not just 1 and 5.