sol2.c File Reference

Problem 1 solution More...

#include <stdio.h>

Include dependency graph for sol2.c:

Functions
int	main ()
	Main function.

Detailed Description

Problem 1 solution

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3,5,6 and 9. The sum of these multiples is 23. Find the sum of all the multiples of 3 or 5 below N.

This solution is based on the pattern that the successive numbers in the series follow: 0+3,+2,+1,+3,+1,+2,+3.

Function Documentation

main()

int main

(

void

)

Main function.

{
    int n = 0;
    int sum = 0;
    scanf("%d", &n);
 
    int terms = (n - 1) / 3;
    sum += ((terms) * (6 + (terms - 1) * 3)) / 2;  // sum of an A.P.
    terms = (n - 1) / 5;
    sum += ((terms) * (10 + (terms - 1) * 5)) / 2;
    terms = (n - 1) / 15;
    sum -= ((terms) * (30 + (terms - 1) * 15)) / 2;
 
    printf("%d\n", sum);
}