My solution to CS50x Problem Set 1 - Credit. Feel free to critique or ask questions.

credit.c

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

int main(void)
{
//gets a valid number from the user
   long long int credit_card;
   do
   {
       credit_card = get_long_long("Number: ");
   }
   while(credit_card < 0);
   //printf("%lld\n", credit_card); 


   int digits = 0;
   long long int number = credit_card;
   //printf("%lld", number);

   for (digits = 0; number/10 > 0; digits++)
   {
       number = number/10;
   }
    digits++;
    //printf("%d\n", digits);

// checks if card is a valid length
    if (digits != 13 && digits != 15 && digits != 16)
    {
        printf("INVALID\n");
        return 0;
    }

//stores every of the credit card as a variable
    int d1 = ((credit_card/(1)) % 10);
    int d2 = ((credit_card/(10)) % 10);
    int d3 = ((credit_card/(100)) % 10);
    int d4 = ((credit_card/1000) % 10);
    int d5 = ((credit_card/10000) % 10);
    int d6 = ((credit_card/100000) % 10);
    int d7 = ((credit_card/1000000) % 10);
    int d8 = ((credit_card/10000000) % 10);
    int d9 = ((credit_card/100000000) % 10);
    int d10 = ((credit_card/1000000000) % 10);
    int d11 = ((credit_card/10000000000) % 10);
    int d12 = ((credit_card/100000000000) % 10);
    int d13 = ((credit_card/1000000000000) % 10);
    int d14 = 0;
    int d15 = 0;
    int d16 = 0;
    if (digits == 15)
    {
        d14 = ((credit_card/10000000000000) % 10);
        d15 = ((credit_card/100000000000000) % 10);
    }
    if (digits == 16)
    {
        d14 = ((credit_card/10000000000000) % 10);
        d15 = ((credit_card/100000000000000) % 10);
        d16 = ((credit_card/1000000000000000) % 10);
    }
    
//finds the digits of each second multiplied by 3, adds them together, and stores it
    int sumd2 = ((d2*2) % 10)+(((d2*2)/10) % 10);
    int sumd4 = ((d4*2) % 10)+(((d4*2)/10) % 10);
    int sumd6 = ((d6*2) % 10)+(((d6*2)/10) % 10);
    int sumd8 = ((d8*2) % 10)+(((d8*2)/10) % 10);
    int sumd10 = ((d10*2) % 10)+(((d10*2)/10) % 10);
    int sumd12 = ((d12*2) % 10)+(((d12*2)/10) % 10);
    int sumd14 = ((d14*2) % 10)+(((d14*2)/10) % 10);
    int sumd16 = ((d16*2) % 10)+(((d16*2)/10) % 10);

//adds together the digits of every second number
    int sum1 = sumd2+sumd4+sumd6+sumd8+sumd10+sumd12+sumd14+sumd16;
//adds together the digits of the remaining numbers
    int sum2 = d1+d3+d5+d7+d9+d11+d13+d15;
//adds those two together (why? because I didn't want to get confused)
    int check = sum1+sum2;
//finds the last value of that overall sum
    int checksum = (check % 10);
//checks to see if last value is 0 
    if (checksum != 0)
    {
        printf("INVALID\n");
        return 0;
    }
    //printf("%d\n", checksum);

    if (digits == 15)
    {
        if (d15 == 3)
        {
            if (d14 == 4 || d14 == 7)
            {
                printf("AMEX\n");
                return 0;
            }
            else
            {
                printf("INVALID\n");
                return 0;
            }
        }
        else
        {
            printf("INVALID\n");
            return 0;
        }
    }
    else if (digits == 13)
    {
        if (d13 == 4)
        {
            printf("VISA\n");
            return 0;
        }
        else 
        {
            printf("INVALID\n");
            return 0;
        }
    }
    else if (digits == 16)
    {
        if (d16 == 4)
        {
            printf("VISA\n");
            return 0;
        }
        else if (d16 == 5)
        {
            if (d15 == 1 || d15 == 3 || d15 == 3 || d15 == 4 || d15 == 5)
            {
                printf("MASTERCARD\n");
                return 0;
            }
            else
            {
                printf("INVALID\n");
                return 0;
            }
        }
        else 
        {
            printf("INVALID\n");
            return 0;
        }
    }
}