Formula expression from task?

Question

Algorithms

Problem: given a segment of numbers [A, B]. We need to find the number of those numbers that are divisible by 3 without a remainder.
How to express the formula for this problem. If you just sort through the numbers - time limit.

Answer 1

Lord, as much as possible. Well, at least elementary learn how to work with a search!
Take A, A+1, A+2. See which of them is divisible by 3. Let this number be equal to C. Then take all the numbers C + 3, C + 2 * 3, C + 3 * 3 .... C + N * 3 less than B. That's it. What is the time limit??
Well, you can also divide B-.A by three, if you are only interested in the number.