The 10% Rule RULES! Part One

 

You can calculate any percentage using the 10% rule.

 

No need to remember any rules, formulae or part/whole/percent triangles.

 

Example 1: What is 15% of 40?

 

 

Start by finding 10%.

 

To find 10%,

chop off the "0" in 40 and you have...

 

4

 

 

5% is half of 10%. Half of 4 is 2, so you need another

 

2

 

4 + 2 = 6

 

 

DONE! Work SMART not HARD

 

 

Example 2: What is 15% of 42?

 

 

There's no zero to chop!

 

Think of 42 as "42 decimal" or "42." Move that decimal one place to the left and you have 4.2

 

This is your 10%.

 

5% is half of 10%. Half of 4.2 is 2.1, so you need another

 

2.1

 

4.2 + 2.1 = 6.3

 

DONE! Work SMART not HARD

 

 

 

 

 

 

Neat Trick for Squares - Well Some of Them

If you are squaring a number that ends in 5, your answer will end in 25.

There's the early Christmas present...

That's the easy part. What about the hard part?

There isn't one...

2

15

 

You know your answer will end in 25...

 

Now, multiply the first digit (the 1 in 15) by ONE larger (2)

 

1 x 2 = 2

 

Stick the 2 in front of the 25 and you have your answer...

 

225

 

Try squaring 25...

 

You know your answer will end in 25...

 

Now, multiply the first digit (the 2 in 25) by ONE larger (3)

 

2 x 3 = 6

 

Stick the 6 in front of the 25 and you have your answer...

 

625

 

Still not convinced?

 

Try squaring 35...

 

You know your answer will end in 25...

 

Now, multiply the first digit (the 3 in 25) by ONE larger (4)

 

3 x 4 = 12

 

Stick the 12 in front of the 25 and you have your answer...

 

1225

 

 

 

 

 

 

 

Quick Calculations - Use Half and Double

 

 

                                           X        


Work smart, not hard!

Double the...

... to...

 and halve the ...
 
 
 

... to...

 

 

100 X 8 = 800. Done!

 

Use "7" as your "Key"

Try this trick for multiplying two 2 digit numbers under 20. For example...

First off, make sure the larger number is on top.

Then think of the number 7. Well, the shape of it, anyway.

Now ADD 13 and 2 and stick a zero on the end.

Now cover up everything in the left (or 10's) column.

Now ADD 150 and 6 and you get 156

Done!