Sum finder not consistent - c++

Question

My code here finds the sum of some given multiples less than or equal to a certain number. It uses a modified version of a formula I read about on the internet a while ago (the one for finding the sum of all the numbers less than or equal to 100, or 1000 or something- when I wrote my formula while I was waiting to be picked up at the ymca so it might not look like the one from the internet). So for me I used (n+x)(n/x/2), where n is the limit (for example 1000), and x is the multiple you are using (so 1, or 3, or 5). So if n = 1000 and x = 5, it should find the sum of all multiples of 5 less than or equal to 1000).
Sometimes it adds up correctly and sometimes it doesn't.
For example, if I choose 1 and 2 as the multiples, and 20 as the limit, it prints out 320 (which is correct if you add 1+2+3...+20 and then add to that 2+4+6...+20).
But if I do the multiples of 3 and 5 and 1000 as the limit, it prints out 266,998 (which is wrong according to the internet).
I do not understand why it worked in the first instance but not the second (I have only taken 1 year of high school math, I'll be a sophomore).
Here is the code:
/*
Finds the sum of all inputted multiples below a certain number
For example, it could find the sum of all the multiples of 3 and 5 less than
or equal to 1000
Written By Jay Schauer
*/
//Data Declarations
#include <iostream>
int main()
{
using namespace std;
int a; //Stores the number of multiples being used
cout << "Enter the amount of multiples you would like to use (up to 50
<< endl;
cout << "(for example, enter '2' if you would like to use two multiples,
maybe 3 and 5?)" << endl;
cin >> a;
cout << "Next, you will enter the mutliples you want to use." << endl;
cout << "(for example, if you want to find the sum of the multiples of 3
and\n5 below a given amount, enter 3 as 'multiple 1' and 5 as 'multiple
2')" << endl;
int multiples[50]; //Stores the multiples being used
for (int i = 0; i < a; i++)
{
cout << "Enter 'multiple " << (i + 1) << "'" << endl;
cin >> multiples[i];
}
int limit; //Stores the limit
cout << "Enter the the limit for how high you want to add the multiples
<< endl;
cout << "(for example, you could set the limit to 1000 to find the sum
of the\nmultiples of 3 and 5 (if you entered those) less than and or
equal to 1000)" << endl;
cin >> limit;
int sum(0); //Stores the sum
for (int i = 0; i < a; i++)
{
sum += ((limit + multiples[i]) * (limit / multiples[i] / 2));
}
cout << "The sum is "<< sum << endl;
system("pause");
return 0;
}
EDIT: I believe the problem might lie in the code not in the formula, because using it on multiples of 3 with 21 as the limit causes it to print out 72, not 84 like it should. I am still unsure of the coding error.
EDIT 2: I changed the for loop to this so it hopefully will function when the limit isn't a multiple of the multiple
for (int i = 0; i < a; i++)
{
int max = limit; /*This is done so I can change max in case it isn't
a multiple of the multiple*/
while (max % multiples[i] != 0) max--;
sum += ((max + multiples[i]) * (max / multiples[i] / 2));
}

Answer 1

Change
sum += ((limit + multiples[i]) * (limit / multiples[i] / 2));
to
sum += (limit + multiples[i]) * (limit / multiples[i]) / 2;
As it is, for your example of 3 and 21, you're computing (24 * (7 / 2)) = 24 * 3 = 72 (integer division of 7 by 2 gives 3, and the remainder is lost), but you want to be computing (24 * 7) / 2 = 84.