What a great idea!
the Quick, Approximate, Mental Arithmetic (QAMA) calculator….Mine just arrived. I typed “25 x 37″ and pressed “=”. A short underline cursor flashed away on the bottom left of the screen, without offering an answer. Instead, it demanded an estimate. Like a skilled tutor, it answered my question with its own.
When I entered 100, it asked again. For how could two numbers, each around 30, multiply only to 100? When I tried 400 and even 800, I still got no answer. Only when I tried 900 did the calculator answer my original question and tell me the exact answer (925).
Here’s the whole article.