function calculate(element) {

	var frm = document.calculator;

	var interest, net, gross, deducted;

	interest = evalnum( frm.interest.value );

	if (interest <= 0) {

		alert("You must enter a figure greater than zero.");
		frm.interest.value='';
		frm.interest.focus();
		return;

	} else {

		frm.interest.value = evalpound( interest );
		gross = interest/4*5;
		deducted = gross - interest;
		net = interest;
		frm.gross.value = evalpound( gross );
		frm.deducted.value = evalpound( deducted );
		frm.net.value = evalpound( net );
	}

}