Question

I have three div when I want to use checkbox should I have new div
if I check car1 while car2 unchecked should I have red car
but if check car1 while car2 checked should I have black car

<div id="carblack">
    car black
</div>
<div id="carred"  style="display:none">
    car red
</div>
<div id="carblue"  style="display:none">
    car blue
</div>
<input type="checkbox" id="car1" name="vehicle" value="red" checked/>
<input type="checkbox" id="car2" name="vehicle1" value="blue"  checked/>

$('#car1').change(function () {

    if($(this).attr("checked")){
        document.getElementById("carred").style.display = "block";
        document.getElementById("carblack").style.display = "none";
    } else{
        document.getElementById("carblack").style.display = "block";
        document.getElementById("carred").style.display = "none";
    }
});

$('#car2').change(function () {

    if($(this).attr("checked")){
        document.getElementById("carblue").style.display = "block";
        document.getElementById("carblack").style.display = "none";
    } else{
        document.getElementById("carblack").style.display = "block"; 
        document.getElementById("carblue").style.display = "none";
    }
});

Answer 1

2 radio checked or none of them – black?

$(function(){
    var cars = $('div[id^="car"]'),
        inputs = $('input[type="checkbox"]');

    inputs.change(function(){
        cars.hide();
        if(inputs.filter(':checked').length === 1)
        {
           cars.filter('#car' + $(this).val()).show();
        }
        else {
           cars.filter('#carblack').show();
        }
    });
});

Demo: http://jsfiddle.net/jAxHT/