Need help with a validation step in my program

I have this much compliled and running, but I am stuck on how to place a validation step in the event the user inputs something not within the required input ($10 denominations). For instance if I input 35 I wil get back one 20 and 1 10. I would like to be able to tell the user that ther input was invalid please reenter desired amount.

Code:

---

* This program will take input from the user of an ATM and provide the user with the number
* of $50, $20 and $10 dollar bills the ATM will dispense based on the user's input. It will
* also prompt the user to reenter their amountif they have entered it incorrectly.*/

#include <stdio.h>

void bills_dispensed (int amount_entered)
{
  int amount_remaining;        /* Holds on to the amount_remaining amount through each step of the calculations. */

  printf("\nYour amount of %d dollars is dispensed as follows.\n", amount_entered);

  /* Compute the total number of 50 dollar bills in the amount. */
  printf("50 dollar bills : %d\n", (amount_entered/50));

  /* Compute amount_remaining to be dispensed after dispensing 50 dollar bills. */
  amount_remaining = (amount_entered%50);

  printf("20 dollar bills : %d\n", (amount_remaining/20));
  amount_remaining = (amount_remaining%20);

  printf("10 dollar bills : %d\n", (amount_remaining/10));
}

int main ()
{
  int amount_entered;

  printf("**************************************\n");
  printf("** Welcome to your neighborhood ATM **\n");
  printf("**************************************\n\n");
  printf("This ATM will only distribute $50, $20 and $10 denominations.\n\n");
  printf("Please enter the amount (multiple of 10 dollars)you wish to withdraw :$ ");
  scanf("%d", &amount_entered);
  bills_dispensed(amount_entered);

  return 0;
}

---

Think about the problem for a bit. How should your amount_entered relate to 10? What symbol (which you use in your program!) gives you that information?

Write a validation function that can be used like this:

Code:

---

printf("Please enter the amount (multiple of 10 dollars)you wish to withdraw :$ ");
scanf("%d", &amount_entered);
while (validation_func(amount_entered) != 0) {
    printf("Try again!");
    scanf("%d", &amount_entered);
}
bills_dispensed(amount_entered);

---

So, validation_func() should return 0 if everything is okay (in which case, the while loop won't happen) and non zero if there is a problem. You could also use that returned value in a switch/case to flesh the problem out (eg, case 2: "That number is too high!").

Code:

---

/* Validation step*/

  while (amount_entered %10) {
    printf("Try again, you have entered an incorrect amount! \n\n");
        printf("Please enter the amount (multiple of 10 dollars) you wish to withdrawl : $");
    scanf("%d", &amount_entered);
  }

---