Mental Computation vs Estimating

We use mental computation all the time in our every day lives without even realizing it. For example, if you are at the store shopping on a limited budget, you want to keep track of your spending to ensure you don't go over a certain dollar limit. In your mind, you would add up the dollar amounts of everything you put in your cart. Mental computation is the process used to find an exact answer to a problem using no tools other than your mind. Therefore we must put aside the pen and paper or the calculator and draw upon our amazing minds to help us arrive at the answer.

I have loosely used the terms mental computation and estimating interchangeabley. This is incorrect because they are two distinct processes.  Estimating is getting “close” to the right answer, whereas mental compuation will get you to the exact answer. There is a time and place for both, but understanding these difference are very important.

To illustrate the difference between mental compuation and estimating, follow my thought process as I personally had the revaltion that there is a difference between the two process. I visited a webpage Valuing Mental Computation Online . This website asked a simple question: how many 45 cent stamps can you buy with ten dollars. In my typical thought process, I mentally rounded the stamps to 50 cents each, and then considered this to be two stamps per dollar. If I have ten dollars, then I can buy 20 stamps. This is where I left my answer, and thus, where I realized I had a lazy throught process. I failed to consider the balance of 5 cents per stamp and how many that would equate to, which is an additional two stamps. I clicked on the link for the answer, and much to my surprise, I was not right in my 20 stamp guess. See the image below for how the mental computation may be correctly executed: