Untitled

<!DOCTYPE html>
<html>
<head>

<script type="text/javascript">

function evthis() {
    var exp = document.getElementById("ev").value;
    var spl = exp.split("=");
    var rev = eval(spl[0]);
    var result = parseInt(spl[1]);

    if (rev == result) {
        document.getElementById("result").innerHTML = "Korrekt!";
    } else {
        document.getElementById("result").innerHTML = "Feil...";
    }
}

</script>

<style>

* {
    font-size: 28px;
    font-family: Arial;
    padding: 10px;
}

body {
    text-align: center;

}

#ev {

}

</style>

</head>

<body>

<form>
<input type="text" id="ev" /> <input type="button" value="Valider" onclick="evthis()" />
</form>

<div id="result"></div>

</body>
</html>