sorted search increase performance

Question

Running through exercises on testdome...currently looking at https://www.testdome.com/for-developers/solve-question/9877

Implement function CountNum

Answer 1

This passed all 4 tests:

public static int CountNumbers(int[] sortedArray, int lessThan)
{
    int val = Array.BinarySearch(sortedArray, lessThan);
    return val < 0 ? ~val : val;
}

As others have said, they expect you to use Array.BinarySearch which I didn't realise until read the 2nd hint.