function calculateA() {
	var mortgage = new Number(document.calculator.mortgage.value);
	var card = new Number(document.calculator.card.value);
	var car = new Number(document.calculator.car.value);
	var other = new Number(document.calculator.other.value);

	var a = mortgage + card + car + other;
	
	if(isNaN(a)) {
		alert("All entries must be numbers");
		document.calculator.a.value = "";
	}
	else
		document.calculator.a.value = a;
	
}

function calculateB() {
	var salary = new Number(document.calculator.salary.value);
	var bonus = new Number(document.calculator.bonus.value);	
	var income = new Number(document.calculator.income.value);
	var alimony = new Number(document.calculator.alimony.value);
	
	var b = Math.round((salary + bonus + income + alimony) / 12);
	
	if(isNaN(b)) {
		alert("All entries must be numbers");
		document.calculator.b.value = "";
	}
	else
		document.calculator.b.value = b;
}

function calculateR() {

	var a = new Number(document.calculator.a.value);
	var b = new Number(document.calculator.b.value);	
	
	if(a=="" || b=="")
	alert("You have not completed the form");
		
	var r = a / b;
	document.calculator.ab.value = Math.round(100 * r) + "%";
}