Help me Out!

See more:

C++

Hey out there, I am new to C++ and I was asked to write a program that reads/gets customer details like names, purchase amount and tax amount. The program is then to calculate the sales tax(tax amount * purchase amount) and amount due(sales tax + purchase amount). I wrote the code like the one below but it rejects the tax amount for the first time and then it accepts it for the second time. Then after that, the program does not compute the expected results correctly. Please help me out!

#include <iostream>

using namespace std;

void tax(void)
{
	char custName[100];
	double purchaseAmnt;
	int taxAmnt;

	cout << "\nCustomer Names : ";
	cin >> custName;

	system("cls");
	cout << "\nPurchase Amount : RM ";
	cin >> purchaseAmnt;

	system("cls");
	cout <<"	TAX MENU\n\n0% - Tax Exempt\n3% - State Sales Tax";
	cout <<"\n5% - Federal Sales State Tax\n7% - Special Sales Tax";
	cout << "\n\nTax Amount.e.g. 3 for 3% : ";
	cin >> taxAmnt;
	
	double computedTax = taxAmnt/100;
	double salesTax = (purchaseAmnt * computedTax);
	double amountDue = purchaseAmnt + salesTax;

	do 
	{
		cout<<"Please enter the correct percentage rate : ";
		cin >> taxAmnt;
	}while ((taxAmnt!=0) && (taxAmnt!=3)&& (taxAmnt!=5) && (taxAmnt!=7));

	switch(taxAmnt)
	{
	case 0:	//Tax Exempt(0%)
		{			
			system("cls");
			cout << "\nNAME            : " << custName <<endl;
			cout << "PURCHASE AMOUNT : " << purchaseAmnt <<endl;
			cout << "TAX AMOUNT      : " << taxAmnt <<"%"<<endl;
			cout << "SALES TAX       : RM " << (purchaseAmnt * computedTax) <<endl;
			cout << "AMOUNT DUE      : RM " << amountDue <<endl;
			break;
		}

	case 3:	//State Sales Tax(3%)
		{
			system("cls");
			cout << "\nNAME            : " << custName <<endl;
			cout << "PURCHASE AMOUNT : " << purchaseAmnt <<endl;
			cout << "TAX AMOUNT      : " << taxAmnt <<"%"<<endl;
			cout << "SALES TAX       : RM " << salesTax <<endl;
			cout << "AMOUNT DUE      : RM " << amountDue <<endl;
			break;
		}

	case 5: //Federal and State Sales Tax(5%)
		{
			system("cls");
			cout << "\nNAME            : " << custName <<endl;
			cout << "PURCHASE AMOUNT : " << purchaseAmnt <<endl;
			cout << "TAX AMOUNT      : " << taxAmnt <<"%"<<endl;
			cout << "SALES TAX       : RM " << salesTax <<endl;
			cout << "AMOUNT DUE      : RM " << amountDue <<endl;
			break;
		}

	case 7:	//Special Sales Tax(7%)
		{
			system("cls");
			cout << "\nNAME            : " << custName <<endl;
			cout << "PURCHASE AMOUNT : " << purchaseAmnt <<endl;
			cout << "TAX AMOUNT      : " << taxAmnt <<"%"<<endl;
			cout << "SALES TAX       : RM " << salesTax <<endl;
			cout << "AMOUNT DUE      : RM " << amountDue <<endl;
			break;
		}
	}
}