I want to build an calculator calculates Apt rates

[leonidassavvides]

I want to build an calculator calculates Apt rates (3 adult apt: 2adults=euro41 _ additional adult=euro6 (1 adult same as two) AND air/condition euro6/apt/day), below code is correct ?
Form At: http://www.holidayaptsaspasia.com/bookingapt.html

head:

<script type="text/javascript">
function calc() {
 if (document.book.ac.checked == true) {
    ac = 6;
 } else {
    ac = 0; 	
 }
 document.book.euro.value = document.book.days.value * (floor((document.book.adults.value)/3) * (47+ac) + 41 + ac) ;
}
</script>

body:

          <tr>
            <th width="164" height="35" align="right" valign="middle" scope="col"><div align="right">No.
              of Apts Interested:</div></th>
            <th width="361" align="center" valign="middle" scope="col"><table width="100%" border="0" cellspacing="0" cellpadding="3" summary="javascript calculator euros total">
              <tr>
                <td width="12%" align="left" valign="middle"><input name="aptno" type="text" id="aptno" value="1" size="1" maxlength="1" onchange="calc();" /></td>
                <td width="35%" align="right" valign="middle">No. of Days to Stay:</td>
                <td width="15%" align="left" valign="middle"><input name="days" type="text" id="days" value="0" size="2" maxlength="2" onchange="calc();" /></td>
                <td width="15%" align="right" valign="middle" bgcolor="#CCCCCC" class="text_normal"><strong>Total(€):</strong></td>
                <td width="23%" align="left" valign="middle" bgcolor="#CCCCCC"><input name="euro" type="text" disabled="disabled" class="text_normal_bold_red" id="euro" value="0" size="6" maxlength="6" /></td>
              </tr>
            </table></th>
          </tr>
          <tr>
            <th height="25" align="center" valign="middle" scope="col">(3 adults
              per Apt)</th>
            <th align="center" valign="middle" scope="col"><table width="100%" border="0" cellspacing="0" cellpadding="3" summary="javascript calculator euros total">
              <tr>
                <td width="25%" align="right" valign="middle">No. of Adults:</td>
                <td width="16%" align="left" valign="middle"><input name="adults" type="text" id="adults" value="2" size="2" maxlength="2" onblur="calc();" /></td>
                <td width="27%" align="right" valign="middle">Air Contition:</td>
                <td width="32%" align="left" valign="middle"><input name="ac" type="checkbox" class="text_normal" id="ac" onblur="calc();" checked="checked" /></td>
              </tr>
            </table></th>
          </tr>

[leonidassavvides]

I must use onblur or onchange to the js calc ?