Is your answer sensible? Always ask yourself this. Estimating what, roughly, your answer should be is a good check as to whether you have gone about your calculation in the right way and whether you have avoided making any mistakes in your arithmetic.

A simple example:

You leave home by car at 8am to visit a friend who lives 578 miles away. You do your calculation and work out that it will take you one hour.

Is that a reasonable answer?    No, it certainly isn't!

The journey of 578 miles couldn't possibly take only one hour by car.


The first thing to do when you make an estimate is:

1.  Make some approximations

Round numbers are easier to deal with. For example, we can round up 578 miles to 600 miles.


2. Do your calculations with the easier numbers

Now you need to think how fast you'll be driving. A round figure could be 60 miles per hour. Use that figure.  If you think that is a bit fast for your driving, you can add more time onto your final estimate to allow for that.


600 miles at 60 miles per hour will take 10 hours – a lot longer than your first answer. Add on an hour for lunch. This makes your estimated time of arrival 7pm.



We often call this a 'back of an envelope' calculation. This is when:


         we don't need an exact figure


         it isn't possible to make an accurate calculation, as in the case of a car journey.


When estimating:

         make your numbers easier to handle by rounding them up or down

         make an estimate for an average amount.


This technique will give you a rough idea of what you need to know.

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