ThreeAndFive (Math)

Question: write an algorithm to find sum of numbers which are smaller than N and divisible by 3 or 5

Example:
N = 9 => 3 + 5 + 6 = 14
N = 10 => 3 + 5 + 6 + 9 = 23

Idea: Arithmetic progression and inclusion-exclusion principle.
The sum of 3's and 5's = The sum of 3's+ The sum of 5's - The sum of 15's. Take care the question asks for smaller than, so 9 does not count if the input is 9.

Time: O(1) Space: O(1)

Code:

 public class ThreeAndFive {  
      public ThreeAndFive()  
      {  
           int input1=9;  
           int input2=10;  
           int input3=16;  
           int input4=100;  
           System.out.println(threeAndFive(input1));  
           System.out.println(threeAndFive(input2));  
           System.out.println(threeAndFive(input3));  
           System.out.println(threeAndFive(input4));  
      }  
      public int threeAndFive(int n)  
      {  
           n--;  
           int numOf3=n/3;  
           int numOf5=n/5;  
           int numOf15=n/15;  
           return getSum(0,numOf3*3,3)+getSum(0,numOf5*5,5)-getSum(0,numOf15*15,15);  
      }  
      public int getSum(int first,int last,int step)  
      {  
           int howMany=(last-first)/step+1;  
           int sum=(first+last)*howMany/2;  
           return sum;  
      }  
 }