Question

How to find the sum of all the multiples of 3 or 5 below 1000 in C

Answer 1

#include <stdio.h>
 
unsigned int Sum(unsigned int n) {
    return (n * (n + 1)) / 2;
}
  
int main() {
    int below1000 = 999;
     
    int sum = 3 * Sum((int)(below1000 / 3)) + 5 * Sum((int)(below1000 / 5)) - (3 * 5) * Sum((int)(below1000 / 15));
    
    printf("sum = %d", sum);
}
  
  
  
  
/*
run:
  
sum = 233168
  
*/

 

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Answer 2

#include <stdio.h>
 
int main() {
    int sum = 0;
  
    for (int i = 3; i < 1000; i++) {
        if (i % 3 == 0 || i % 5 == 0) {
            sum += i;
        }
    }
      
    printf("sum = %d", sum);
}
  
  
  
  
/*
run:
  
sum = 233168
  
*/

 

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