A Rough Guide to Everyday Mental Calculation!

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A Rough Guide to Everyday Mental Calculation!

(c) Copyright 2004 by Murdo Macleod

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We all need to check a bill from time to time.

The problem is, how do you do it when you're in a rush, when
you're under pressure, and when you don't have a calculator
handy?

Actually, it's not at all difficult if you know what to look
for. Here's how to do it...

Example 1
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Suppose you are ordering some building supplies and you want
to check that the price is about right before you open your
check book.

You order 213 posts at $5.85 per post.

These numbers can be approximated to 200 posts at $6 per
post.

Multiplying 200 and 6 is easy: the expected bill should be
about $1200.

Basically you just look at the figures and round them to the
nearest convenient unit. (That's why we approximated $5.85
to $6).

Example 2
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Similarly for 330 pieces of turf @ 65 cents each,
just find 300 × 70 = 21000 cents = $210.

--- TIP --------------------------------------------------

When multiplying numbers ending in zeros e.g. 10 x 100 =
1000: you always end up with the number of zeros added.
So 1000 x 30,000 = 30,000,000 (7 zeros)

--- TIP --------------------------------------------------

Example 3
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If there are more items in the bill you can still get a
rough answer:

62 planks @ $2.85 each,
28 joists @ $6.99 each.
Nails: $8

Get 60 × 3 = 180 for the planks,
and 30 × 7 = 210 for the joists.
The nails are $8, let's say $10.

So the rough total is: 180 + 210 + 10 = $400

Now who needs a calculator?

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Excerpt from 'Fun With Figures Volume 2'. Discover how to
perform everyday calculations the easy way - without having
to rely on your electronic friend!.
http://FunWithFigures.com/vol2/

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