How to solve the "function teens()" ?

Need some clue on what it means. Trying to translate a number between 10 and 99 into its word. Here is my code and the confusion pointed out.

// Name:

// Date:

// Purpose: To convert any number from 10 - 99 to its word representation

// This line makes the button, btnConvert wait for a mouse click

// When the button is clicked, the convertNumber function is called

btnConvert.addEventListener(MouseEvent.CLICK, convertNumber);

// These lines make the textinputs wait for a mouse click

// When any of these components are clicked, the clearLabels function is called

txtinNumber.addEventListener(MouseEvent.CLICK, clearLabels);

// Declare Global Variables

var num:int;         // number from 10 - 99

var tensDigit:int;   // the tens digit

var onesDigit:int;   // the ones digit

// This is the convertNumber function

// e:MouseEvent is the click event experienced by the button

// void indicates that the function does not return a value

function convertNumber(e:MouseEvent):void

{

getData();

if (num < 10 || num > 99){

lblOutput.text = "Invalid number. Enter a number between 10 and 99 inclusive.";

}

else{

lblOutput.text = "";

if (num >= 20) {

tensDigit = Math.floor(num / 10);

onesDigit = num % 10;

tens();

ones();

}

else{

tensDigit = Math.floor(num / 10);

onesDigit = num % 10;

teens();

}

}

}

// This is the getData function

// It gets the number from the user

function getData()

{

// complete the code here

num = int(txtinNumber.text);

}

// This is the tens function

// It outputs the word representation of 20, 30, 40,..,90

function tens()

{

//write the code here

lblOutput.text = onesDigit.toString();

}

// This is the ones function

// It outputs the word representaion for any number from 1 - 9 inclusive

function ones()

{

// write the code here

lblOutput.text = tensDigit.toString() + onesDigit.toString();

}

// This is the teens function

// It outputs the word representation for any number from 10 - 19 inclusive

function teens()

{

// write the code here

lblOutput.text = teens();

}

// This is the clearLabels function

// e:MouseEvent is the click event experienced by the textInput

// void indicates that the function does not return a value

function clearLabels(e:MouseEvent):void

{

     lblOutput.text = "";

}