generate your own captcha vs generate image

Question

is it easier to get hacked when you generate your own captcha code than when you use image captcha?

and how can I add alphabet to this code. this is the captcha that is created on your own.

<script type="text/javascript"> 
//Generates the captcha function     
var a = Math.ceil(Math.random() * 99)+ ''; 
var b = Math.ceil(Math.random() * 99)+ '';        
var c = Math.ceil(Math.random() * 99)+ '';   
var d = Math.ceil(Math.random() * 99)+ '';   
var e = Math.ceil(Math.random() * 99)+ '';   
var code = a + b + c + d + e; 
document.getElementById("txtCaptcha").value = code; 
document.getElementById("txtCaptchaDiv").innerHTML = code;   
</script>

Answer 1

Yes,
the purpose of captcha is to differentiate between Human traffic and bots.
One will require no time to code a bot which can read text captcha.
on the other side it will require too much image processing, computing and a smart algorithm to decode the text in image.
so it is difficult to crack.