Project Euler problem 1 Solution in C, python and using pen and paper

Q:-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 1000.

Solution using C:

#include<stdio.h>

int main(){
int a,sum=0;

//going through all numbers from 1-1000
for (a=1;a<1000;a++){
if (a%3==0 || a%5==0){//check whether each number is divisible by 3 or 5
sum +=a;} //if so add that number to sum
}

printf("sum is : %d", sum);
getchar();

}


Solution using Python(3):

sum=0
for i in range (1000):
   if i%3==0 or i%5==0:
      sum+=i
print(sum)


Solution using pen and paper:

using the formula  S_n=n*(a+l)/2 use for both numbers divisible by 3 and 5 seperatly and add the sum.