Number of Guests: <input id="guests" /><br/>
Number of Hours: <input id="hours" /><br/>
Going to Oakbluffs?<input type="checkbox" id="isOB" value="isOB"><br/>

<button onClick="calc()">Calculate Charter Cost</button>
<p>The charter fee would be $<span id="cost">0</span></p>

<script>
function calc()
{
  var numGuests = document.getElementById("guests").value;
  var numHours = document.getElementById("hours").value;
  var isOB = document.getElementById("isOB").checked;
  var cost = 0;

  if (numGuests > 6)
  {
    alert("Maximum number of guests is 6.");
    return; // exit the function now! don't calculate
  }

  cost = (275 * numHours) + (15 * numGuests);
  
  if (isOB == true)
  {
    cost = cost + 20;
  }

  document.getElementById("cost").innerHTML = cost;
}
</script>