Project Euler 1 : To find the sum of all multiples of 3 or 5 below n.

euler1.c

#include<stdio.h>

int main(){
  int n ;
  int sum ;
  scanf("%d",&n) ;

  int three = (n-1) / 3 ; // no of 3 multiples below n
  int five = (n-1) / 5 ; // no of 5 multiples below n
  int fifteen = (n-1) / 15 ; // no of 15 multiples below n

  sum = 3 * (three * (three+1)) / 2 ; // finding sum of all multiples of 3 below n
  sum += 5 * (five * (five+1)) / 2 ; // finding sum of all multiples of 3 below n
  sum -= 15 * (fifteen * (fifteen+1)) / 2 ; // finding sum of all multiples of 3 below n

  printf("%d\n",sum);
}