The Summinator!

You could do this using Javascript.
1) create a textfield
2) create a submit button
3) define two Javascript variables: a running tally, and a click-counter
4) give the button an onclick event that calls a function that works like this:

read the value from the text field
convert it to a numeric value
if that value is 0 {
if the counter != 0 {

display the tally and the average (which equals tally over counter)

}else{
either display an error message, or 0 as both the tally and average - to your taste.

}
reset both the tally and the counter to 0

}else{
add the value to the tally
increment the counter by 1

}

And here's a working example of how to do it:

The Summinator!

Add Value

You can even make it a little nicer by having it output the sum and average each time a number is entered, rather than waiting for the zero input (which would still reset things). That actually simplifies the code too, and would look like this:

...

if(isNaN(num)){ // make sure they entered a number
alert("Please enter a numeric value.");

}else{
if(num != 0){
tally += num;
counter ++;
sum = tally;
avg = tally / counter;

}else{
sum = avg = tally = counter = 0;

}

sumfield.innerHTML = "The sum is " + sum;
avgfield.innerHTML = "The average is " + avg;

}

...

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