function check48(n) {
   if ("" === n) {return true;}
   return n >= 48;
} 

function check6(n) {
   if ("" === n) {return true;}
// use the modulus operator '%' to see if there is a remainder    
if (n % 6 != 0) {
      alert('Order by half-dozen increments greater than 48');
	  return false;
}    

return true;  
} 
 // Attach your form's onsubmit handler
var f = document.getElementById('erb'); 
f.onsubmit = function() {
var el = document.getElementById('WHT'); 
return (check6(el.value) && check48(el.value));
}