I guess the author of the comment already knows, but let it stay for new people.

for a person who does not know what this code is for, he has no idea what the number 40 is.
in this case, the constant is used for two reasons in bussines case,
1 instead of a comment,
2 if you update the price list, you change it in one place, not in several.

the same if 'd' was 'days' a new person immediately knows what the code is for.