public void calculate(double rate, double hours, double salary) {
		if (rate == 0) {
			rate = salary * (1 / hours) * (1 / WKS_PER_YR);
			rateField.setValue(rate);
		}
		if (hours == 0) {
			hours = (1 / rate) * salary * (1  / WKS_PER_YR);
			hoursField.setValue(hours);
		}
		if (salary == 0) {
			salary = rate * hours * WKS_PER_YR;
			salaryField.setValue(salary);
		}
	}