A Rough Guide to Everyday Mental Calculation!

by Murdo Macleod

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

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

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)

Example 3

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?

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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