function calculateCost() {    
    var tour = document.booking.whichtour[document.booking.whichtour.selectedIndex].value;
    var numAdults = document.booking.adultnumber[document.booking.adultnumber.selectedIndex].value;
    var numChilds = document.booking.childnumber[document.booking.childnumber.selectedIndex].value;
   // var numBP = document.booking.backpackernumber[document.booking.backpackernumber.selectedIndex].value;
    var mydiscount = document.booking.discount;
    var mytotal = document.booking.total;
    
    if(numAdults == '15+' || numChilds == '15+' 
// || numBP == '15+'
) {
        mydiscount.value = 0;
        mytotal.value = 'Call for bonus Discount!';
        return;
    }
    
    if(isNaN(numAdults)) {
        numAdults = 0;
    }
    if(isNaN(numChilds)) {
        numChilds = 0;
    }
  //  if(isNaN(numBP)) {
  //      numBP = 0;
  // }
    
    var theval = 0;
    if(tour == 'BBWT' || tour=='Platypus') {
        theval = 70*numAdults + 30*numChilds 
// + 65*numBP;
    }
    if(tour == 'Whale') {
        theval = 70*numAdults + 30*numChilds 
// + 25*numBP;
    }

    mydiscount.value = theval*0.1;
    mytotal.value = theval - theval*0.1;   
}