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Here's another mental math tip.
Since it's easy to calculate 10%, we can use a shortcut to calculate 20%, 30%, 40%, 50%, etc. Check this out:
20% of a number = (10% of the number) × 2
30% of a number = (10% of the number) × 3
and so on…
For example:
A $390 TV is marked down 60% (or 10% × 6). What's the final price?
First let’s calculate the discount. What is 60% of $390?
We find 10% of $390 = $39 by moving the decimal in $390 one place to the left. We can round $39 to $40 for easier mental math, so let’s say 10% of $390 is about $40.
Now we find 60% of $390: 60% of $390 = (10% of $390) × 6 is about ($40) × 6 = $240
Finally, subtract the $240 discount from $390 to get a final price of about $150. Now that's a bargain.