If you read our last article on Finding Percentages and Values and practiced a little bit, you probably came across a very nice percent to work with, 10%.
Why is 10% such a nice percentage to work with? Because it makes everything so easy, you can do it in your head! In fact, all you have to do is more the decimal point over 1 place value, and you have your new number.
For example: 10% of 25 is 2.5.
10% of 1234 is 123.4
10% of 500 is 50
It's even easier when you see it in action!
How does this help to quickly estimate percentages? Consider this: A store having a big inventory sale. Anything that costs $45 is now 20% off! What is the sale price?
You could do the math as show in Finding Percentages and Values, but 20% is pretty easy to work with; it's twice 10%.
If we figure out 10% in our head, we move the decimal point over 1 point, to show that 10% of $45 is $4.5. Then we double it. $4.5 + 4.5 is $9. So we get $9 off the original price of $45. 45  9 is 46. You can do all this in your head!
What is 5% of 80? Since 5% is half of 10%, this is just as easy! 10% of 80 is 8; half of 8 is 4.
You can also combine the halfs and doubling to perform quick percentages!
The normal price of a new computer is $700. There is a big sale for 25% off. How much is the new computer?
Step 1: 10% of $700 is $70.
Step 2: It's 25% off, so we double the $70, and that gives us $140.
Step 3: The last 5%  if 10% is $70, then 5% must be half of that, $35.
Step 4: Add the 20% ($140) and 5% ($35), and we see that the computer is $175 less than the normal price! And with practice, we can do all this in our heads!
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