How can I fix the error : Floating point exception (core dumped) in my program?

Question

Several weeks ago, I started to do the CS50 course on EdX to learn a little bit about Computer Science then I started to code in C, so if the program that follows is not really well made, that’s normal. So basically, when I try to run my program, I get the error message : Floating point exception (core dumped). By looking on the Internet, I understood that it means that there is some expression dividing value by zero in my program, but I cannot figure out where the error comes from. So, please help. And thank you very much in advance for your response.

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    //Prompt for input
    long cardNum = get_long("Enter your card number : ");

    //Calculate the checksum
    int totalneedx2 = 0;
    int total;
    int needx2;

    for (int n = 100; n < cardNum; n = n * 100)
    {
        needx2 = cardNum % n / n * 10;
        if (needx2 >= 5)
        {
            int unit;
            needx2 = needx2 * 2;
            unit = needx2 % 10;
            needx2 = needx2 / 10;
            total = needx2 + unit;
        }
        else if (needx2 < 5)
        {
            needx2 = needx2 * 2;
            total = needx2;
        }
    totalneedx2 = total + totalneedx2;
    needx2 = 0;
    }

    int nox2;
    int totalnox2 = 0;
    for (int n = 10; n < cardNum; n = n * 100)
    {
        nox2 = cardNum % n / n * 10;
        totalnox2 = totalnox2 + nox2;
        nox2 = 0;
    }
    int totalChecksum = totalnox2 + totalneedx2;

    if (totalChecksum % 10 == 0)
    {
        if (cardNum >= 1000000000000)
        {
            if (cardNum / 1000000000000 == 4)
            {
                printf("VISA\n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else if (cardNum >= 100000000000000)
        {
            if (cardNum / 10000000000000 == 34 || cardNum / 10000000000000 == 37)
            {
                printf("AMEX/n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else if (cardNum >= 1000000000000000)
        {
            if (cardNum / 100000000000000 == 51 || cardNum / 100000000000000 == 52 || cardNum / 100000000000000 == 53 || cardNum / 100000000000000 == 54 || cardNum / 100000000000000 == 55)
            {
                printf ("MASTERCARD\n");
            }
            else if (cardNum / 1000000000000000 == 4)
            {
                printf ("VISA\n");
            }
        else
        {
            printf("INVALID\n");
        }
        }
    }
    else
    {
        printf("INVALID\n");
    }
}