Someone

<script type="text/javascript">

$(document).ready(function() {
	
	function calculate () {
	
       // USER INPUT
	    	
		var active_vehicles_in_fleet = $('#element_1').val();  
		var average_hourly_driver_wage_including_benefits = $('#element_2').val();
		var average_refueling_volume_per_vehicle_litres = $('#element_3').val();
	
	   // USER RESULTS
	   
		var abcdefg = 10;
	
		$('p.1').replaceWith(abcdefg);	
		
	}
  
    $('#element_3').keypress(function() {
    	
		calculate();
	
	});
});
</script>