Output in one window
The Java standard dictates the use of the "Egyptian braces" style:

if (something) {
    // something...
}

In AvarageCalc():


int avarage = sum / population;
return avarage;



you can just return the calculation:

return sum / population;


Also, it should be spelled average. ;-)
In Output(), you don't need to declare text and then assign it later.  Instead, initialize it where it's first used.  Always try to keep variables as close in scope as possible, which can help keep your code cleaner and easier to maintain.

String text = messageText[0]+ "\n" + listOfNums;