The Count Div problems is classified as a medium difficulty but I found the solution to be quite trivial and easily solvable in O(1).

The only thing we need to do is to calculate how many numbers are divisible by a third one in between two value.

Thus, the solution:

```
function solution(A, B, K) {
// write your code in JavaScript (Node.js 8.9.4)
if (A === B) {
if (A % K === 0) {
return 1
}
else {
return 0
}
}
leftLimit = Math.ceil(A/K)
rightLimit = Math.floor(B/K)
return (rightLimit - leftLimit + 1)
}
```

The report is here.