Arrays

Hey i am making a quiz which is going to have 20 questions and i have about 40 question in the questions array. Now can you please tell me how to make a variable which will hold the 20 random question so

for eg

var arrQuestion:Array = [ 40 questions in here separated by commas]
var randQuestions:int = arrQuestion.length-1;

but that just sets randQuestions to the arrQuestion’s length now how do i set the variable randQuestion with 20 questions that are randomly picked from arrQuestion.