Calculate and return the value of 3*f+5*b-15*fb in a FizzBuzzProblem

Given: A number is considered fizz if it is divisible by 3. It is considered buzz if it is divisible by 5. It is considered fizzbuzz if it is divisible by both 3 and 5. A fizzbuzz is neither fizz nor buzz. Given two numbers n1 and n2 such that n2>n1, let f be the number of fizz, b be the number of buzz and fb be the number of fizzbuzz between n1 and n2(both n1 and n2 are included).

Calculate and return the value of 3*f+5*b-15*fb?

MyApproach:

I used all conditions given in the question but I am confused how to use this condition "A fizzbuzz is neither fizz nor buzz"

I am getting wrong output may be because of this Can Anyone guide me how to solve the problem.

public int calcFB (int n1, int n2) 
{
    int f=0;
    int b=0;
    int fb=0;
  for(int i=n1;i<=n2;i++)
  {
     if((i%3==0)&&(i%5==0))
     {
       fb++;
     }
     else if(i%3==0)
     {
        f++;
     }
     else if(i%5==0)
     {
        b++;
     }


  }
   return 3*f+5*b+15*fb;

}

For the Input:

 Parameters      Actual Output  Expected Output
'1' '21'         56             18