: 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

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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 .85 per post.
These numbers can be approximated to 200 posts at per post.
Multiplying 200 and 6 is easy: the expected bill should be about 00.
Basically you just look at the figures and round them to the nearest convenient unit. (That's why we approximated .85 to ).
Example 2 ---------
Similarly for 330 pieces of turf @ 65 cents each, just find 300 × 70 = 21000 cents = 0.
--- 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 ---------
If there are more items in the bill you can still get a rough answer:
62 planks @ .85 each, 28 joists @ .99 each. Nails:
Get 60 × 3 = 180 for the planks, and 30 × 7 = 210 for the joists. The nails are , let's say .
So the rough total is: 180 + 210 + 10 = 0
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!.
FunWithFigures.com/vol2/
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