The correct answer is: 500.
The product of the rise and going of a stair is approximately kept equal to 500. This is because this ratio provides a comfortable and safe walking experience for the users. A rise that is too high or a going that is too short can make the stairs difficult to climb, while a rise that is too low or a going that is too long can make the stairs feel unstable. The 500 rule ensures that the stairs are neither too steep nor too shallow, and that they provide a comfortable and safe walking experience for all users.
The rise is the vertical distance between one tread and the next, while the going is the horizontal distance between the front edge of one tread and the front edge of the next. The 500 rule states that the product of the rise and going should be approximately equal to 500. This means that the rise and going should be in a ratio of about 1:2.
There are some exceptions to the 500 rule. For example, in some cases, the rise and going may be allowed to be slightly different from 500 if this is necessary to accommodate the needs of people with disabilities. However, the 500 rule is a good general guideline for designing stairs that are comfortable and safe for all users.