Edit in JSFiddle

var log = function(content){
    $('<p/>').html(content).appendTo('#log');
}

function doTask1(add, multiply){
    setTimeout(function(){
        var rslt = Math.floor(Math.random() * 10);
        log("Task 1 returned " + rslt);
        if(rslt >= 5)
            add(rslt);
        else
            multiply(rslt);
    }, 500);
}
    
function addTask(i, next){
    setTimeout(function(){
        var rslt = Math.floor(Math.random() * 10);
        log("addTask finished with " + rslt);
        next(i + rslt);
    }, 500);
}
    
function multiplyTask(i, next){
    setTimeout(function(){
        var rslt = Math.floor(Math.random() * 10);
        log("multiplyTask finished with " + rslt);
        next(i * rslt);
    }, 500);
}
    
$(function(){
    doTask1(function(rslt){
        addTask(rslt, function(final){
            log("All tasks finished with " + final);
        });
    }, function(rslt){
        multiplyTask(rslt, function(final){
            log("All tasks finished with " + final);
        });
    });
});

<div id="log"></id>