Question

How to find the sum of all the multiples of 3 or 5 below 10 in C

Answer 1

#include <stdio.h>

int main() {
    int sum = 0;
     
    for (int i = 3; i < 10; i++) {
        if (i % 3 == 0 || i % 5 == 0) {
            printf("%d\n", i);
            sum += i;    
        }
    }
   
    printf("sum = %d", sum);
}
 

 
 
/*
run:
 
3
5
6
9
sum = 23
 
*/

 

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Answer 2

#include <stdio.h>
 
unsigned int Sum(unsigned int n) {
    return (n * (n + 1)) / 2;
}
  
int main() {
    int below10 = 9;
     
    unsigned int sum = 3 * Sum((int)(below10 / 3)) + 5 * Sum((int)(below10 / 5)) - (3 * 5) * Sum((int)(below10 / 15));  
     
    printf("sum = %d", sum);
}
  
  
  
  
/*
run:
  
sum = 23
  
*/

 

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