How to use Javascript to ask the user random math questions?

function ask() {
    var a = Math.floor(Math.random() * 10) + 1;
    var b = Math.floor(Math.random() * 10) + 1;
    var op = ["*", "+", "/", "-"][Math.floor(Math.random()*4)];
    return prompt("How much is " + a + " " + op + " " + b + "?") == eval( a + op + b);
}

var questions = [ask(), ask(), ask(), ask(), ask()],
    total = questions.length,
    correct = questions.filter(Boolean).length;

alert( "You got "+correct+"/"+total+" correctly");

1. Define possible operations in an array
2. Take randomly one of these operations
3. Take randomly two numbers between some limits (e.g.: 0-30) (check for unnacceptable cases like 10 / 0)
4. Compare user input with the computed result. If float, apply a small tolerance.

The implementation couldn't be easier.

EDIT Hint: construct an object that contains all the functions and take randomly from it. This way you avoid eval():

var operations = {
    '+': function (a, b) {return a + b;},
    '-': function (a, b) {return a - b;},
    '/': function (a, b) {return a / b;},
    'x': function (a, b) {return a * b;}
}

If you're happy with just addition:

function question() {
    this.a = Math.round(Math.random()*10);
    this.b = Math.round(Math.random()*10);
    this.result = this.a + this.b;
    this.checkResult = function(givenResultString) {
        return (""+result == givenResultString);
    }
}

Then you can create a question each time with:

var q = new question();

And check an answer:

var response = q.checkResult(someString) ? "Correct!" : "FALSE!";

The rest of the job is the mechanics of the page, for which you could use a form and a result div.

If you want to add more operations, you would pick a random operator as well as random inputs:

function question() {
    var add(x, y) { return x+y; };
    var subtract(x, y) { return x-y; };
    var multiply(x, y) { return x*y };
    var operators = [add, subtract, multiply];

    this.a = Math.round(Math.random()*10);
    this.b = Math.round(Math.random()*10);
    var operatorIdx = Math.min(Math.floor(Math.random()*4),3);
    this.operator = operators[operatorIdx];
    this.result = operator(this.a,this.b);
    this.checkResult = function(givenResultString) {
        return (""+this.result == givenResultString);
    }
}

I've left division off here, as the rest assumes integers and you'd have to change your initialisation to prevent a division from producing fractional values. The straightforward way would be to initialise a multiply, then swap the result and a.