C Exercises: Sum of all the multiples of 3 or 7 below 100

Last update on August 09 2025 12:47:00 (UTC/GMT +8 hours)

17. Sum of Multiples of 3 or 7 Below 100 Variants

The natural numbers below 10 that are multiples of 3 or 7 are 3, 7, 6 and 9. The sum of these multiples is 25.
Write a C programming to find the sum of all the multiples of 3 or 7 below 100.

C Code:

#include <stdio.h>
int main(void)
{
  int sum3 = 0, sum7 = 0, sum21 = 0;
  int i;
  for (i = 0; i < 100; i++) {
    if (i % 3 == 0) {
      sum3 += i;
    }
    if (i % 7 == 0) {
      sum7 += i;
    }
    if (i % 21 == 0) {
      sum21 += i;
    }
  }
  printf("%d\n", sum3 + sum7 - sum21);
  return 0;
}

Sample Output:

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to list all multiples of 3 or 7 below 100 and then compute their sum.
Write a C program to compute the sum of multiples of 3 or 7 below a user-specified number using recursion.
Write a C program to calculate the sum of multiples of 3 or 7 below 100 using a mathematical formula.
Write a C program to compute and display the sum and count of multiples of 3 or 7 below 100.

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