Please point me in the right direction I've been working on this for 3 days. I'm so frustrated I just need a little direction...please

I'm attempting to modify the C program so that the user inputs the purchase amount. Check the user’s input for validity. In addition to calculating the sales tax for each location,calculate and display the total
sale amount for each location.
Tax Calculations: Del Mar=7.25%,Encinitas=7.50%, La Jolla= 7.75%
/***********************************************************
SR-kf-008, Change Request#1
Kudler Fine Foods

Tax Calculations: Del Mar=7.25%,Encinitas=7.50%, La Jolla= 7.75%
*************************************************************/

#include <stdio.h> // Include standard I/O libraries
#include <stdlib.h> // Include standard libraries

Where are you getting the input?

From the command line (fed into the program)?
Prompt the user & then listen for input?
Read it from a file?

I'm in luck one of the students posted their response. I'll look through their code to see if it can help me create my own. Thx!

/*-----------------------------------------------------------------------------
Program:
Date:
Author:
Class:
Requestor:
Change: Change Request #1

Description: This C program will prompt the user to enter the purchase amount
and will check the user's input for validity. The correct tax
amount and total purchase amount will be calculated on the input
value. The purchase amount, Tax amount, and Total Sale amount
will be displayed for each store location at Kudler Fine Foods.

Tax Rates: Del Mar (7.25%), Encinitas (7.5%), La Jolla (7.75%)
-------------------------------------------------------------------------------*/
#include <stdio.h> // Include standard I/O libraries
#include <stdlib.h> // Include standard libraries
#include <math.h> // Include Math libraries

#define DMTaxRate 7.25 /* Set constant Tax Rate for Del Mar = 7.25% */
#define ENTaxRate 7.50 /* Set constant Tax Rate for Encinitas = 7.50% */
#define LJTaxRate 7.75 /* Set constant Tax Rate for La Jolla = 7.75% */


main()
{
char Purchase[32]; /* Declare program variables */
double Amount,DMTax,ENTax,LJTax;
int i,NV=0;

/* Prompt user for Purchase Amount */
printf("\n\n\tKudler Fine Foods Tax Calculator\n\n");
printf("\n\n\tPlease Enter Amount of Purchase:");
printf("\n\n\t===> ");
scanf("%s", Purchase);

/* Loop - Test each character in Input String (Purchase) */
for(i=0; Purchase[i]; i++)
{
if(Purchase[i] < '0' || Purchase[i] > '9') /* Test for a numeric value (0-9) */
{
if (Purchase[i] == '.') /* If character is a decimal continue */
continue;
else
NV=1; /* Set Non-Numerical Value flag */
break;
}
}

if (NV == 1) /* If Non-Numerical Value print Error Message */
{
printf("\n\n\n\tError! Invalid Purchase Amount [ %s ]\n",Purchase);
fflush(stdin);
printf("\n\n\n\tPress any key to continue...\n\t");
getchar(); /* Pause to allow viewing of output on screen */
return 0;
}


Amount=atof(Purchase); /* Convert Ascii character to Float */
DMTax=(Amount*DMTaxRate)/100; /* Calculate Tax Amount for Del Mar */
ENTax=(Amount*ENTaxRate)/100; /* Calculate Tax Amount for Encinitas */
LJTax=(Amount*LJTaxRate)/100; /* Calculate Tax Amount for La Jolla */

/* Display Tax Calculation output to the screen */
printf("\n\n\tKudler Fine Foods Tax Calculations by Store:\n\n");
printf("\n\tDel Mar Tax on $%.2f = $%4.2f Total = $%4.2f",Amount,DMTax,Amount+DMTax);
printf("\n\tEncinitas Tax on $%.2f = $%4.2f Total = $%4.2f",Amount,ENTax,Amount+ENTax);
printf("\n\tLa Jolla Tax on $%.2f = $%4.2f Total = $%4.2f",Amount,LJTax,Amount+LJTax);

fflush(stdin);
printf("\n\n\n\tPress any key to continue...\n\t");
getchar(); /* Pause to allow viewing of output on screen */
return 0;
}