27 10 2010
A Rough Guide to Everyday Mental Calculation!
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
Actually, it’s not at all difficult if you know what to look
for. Here’s how to do it…
Suppose you are ordering some building supplies and you want
to check that the price is about right before you open your
You order 213 posts at $5.85 per post.
These numbers can be approximated to 200 posts at $6 per
Multiplying 200 and 6 is easy: the expected bill should be
Basically you just look at the figures and round them to the
nearest convenient unit. (That’s why we approximated $5.85
Similarly for 330 pieces of turf @ 65 cents each,
just find 300