ESTIMATING

**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

**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

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

•
we don't need an exact figure

or

•
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.