JavaScript if statement: apply when a certain value changes to another certain value

How do I write an if statement so that it applies only when Value A changes to Value B in a select box? http://ift.tt/1Sa36v7

I have the following select box for "miles" and input box for "residual":

<div class="item_div"><label>Miles:</label>
    <select id="miles">
        <option value="10000">10,000</option>
        <option value="12000" selected>12,000</option>
        <option value="15000">15,000</option>
    </select>
</div>
<div class="item_div">
    <label>Residual: </label>
    <input type="number" id="resP" class="rmVal" value="52"> % 

</div>
<div>
    <label>Result (10000*Residual)</label>
    <input id = "result"></input>

</div>

I would like to add 1 to the miles <input> value if the user changes from 12000 to 10000. If the user changes from 10000 to 15000, I would like to subtract 3. If the user then changes from 15000 to 120000, I would like to +1 and so on.

The residual values for corresponding miles are:
10000 miles: 53%
12000 miles: 52%
15000 miles: 50%

The reason I wanted to make the residual change based on the miles change is that later on, I want the user to be able to manually change the residual value and adjust the residual value based on the miles.

I tried to following code to +1 when the $("#miles") changes from 12000 to 10000 and -2 if it changes from 12000 to 15000 (line 13-26):

if (miles.val() == "12000") {
    miles.change(function() {
        if (miles.val() == "10000") {
            resN = resN +.01;
            resP.val(parseFloat(resP.val())+1);
            calculate();
        }
        else if (miles.val() == "15000") {
            resN = resN -.02;
            resP.val(parseFloat(resP.val())-2);
            calculate();
        }
    })
};

I repeated the if statement for the other miles changes (line 27-55). However, the code only works correctly for the 1st and 2nd miles change. The 3rd time on is messed up. I was wondering how I may correct my code and whether there is an easier way to accomplish this. Thank you very much! I much appreciate it!