This blog entry has a good write up of ten key calculators

Learn to Add and Subtract with a Ten Key Calculator
Equals (=) Is Not the Way to Go

Use the total key (* or T) to sum your numbers

The easiest way to think of a ten-key calculator is as an adding machine (many of which print this nice little tape for you as proof that you did indeed add that staggering column of numbers correctly). Its sole function is to add numbers. Positive numbers, negative numbers, all kinds of numbers. If it's of decent quality, it will even tell you how many numbers!

The item count is a neat feature that can also stupify if you haven't had much experience with a ten-key--or if you just didn't bother to read the instructions that came with your calculator. However, I'll explain all that in a few moments.

Adding numbers on the ten-key

This is not as intuitive as you might think. I recently answered an email where someone asked me if I had it wrong below. Trust me. I don't. I'll include the email and my response below my tutorial to help make my point clear.

Okay, so we want to add some numbers.

6+5+9+8

Now, do I have to tell you to turn on your calculator? (Please don't make me. I'm not sure I can handle the stress.)

Press: [ C/CE (or C) ] [ C/CE (or C) ]

Press: [ 6 ] [ + ] [ 5 ] [ + ] [ 9 ] [ + ] [ 8 ] [ + ]

See your total there on the screen now? I hope it shows 28. If it doesn't, start over at #1 above. Quickly, now! We're moving on here.

Press: [ * (or T) ]

Nothing happened, did it? That's all right. Nothing was supposed to happen. If you had hit the clear button before you pressed the *, your calculator wouldn't have printed the total or concluded the calculation.

Adding and subtracting numbers on the ten-key

Wow, that was easy, agreed? So, now let's change things up a bit. We'll add and subtract some numbers.

6+5-9-8

Press: [ C/CE (or C) ] [ C/CE (or C) ]

Press: [ 6 ] [ + ] [ 5 ] [ + ] [ 9 ] [ - ] [ 8 ] [ - ]

See your total there on the screen now? I hope it shows —6 or 6.— depending how your calculator shows negative numbers. If it doesn't, start over at #1 above. The thing to remember here is that the calculator is going to ADD everything you input with the keys. So if you want to subract something, you have to enter it as a negative number or, if it's easier to remember, think of it as telling the machine you want to add into the total the number 6 you just entered, you want to add in the 5, take away or subtract the 9, and take away or subtract the 8. But you can't tell the machine to do these things with a number until you've entered the number itself! ***[see below for some further clarification]

Press: [ * (or T) ]