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A Rough Guide to Everyday Mental Calculation!
By Dave Blake | July 30, 2007
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)
— TIP ————————————————–
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?
Topics: Strategy |
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