Tricks!

A trick is associated with magic, something happening without an obvious explanation.

Mental arithmetic tricks are just the same. When you first learn them you may not know why they work but they are very useful nevertheless. They are even more useful when you are able to explain why they work and understand their limitations.

Here are some mathematical tricks for you to try ....

Quick Square

Did you know that there is a quick way of squaring a two digit number which ends in 5?
Just multiply the first digit by that number plus one.... stick a 25 after your product and there's your answer .... simple ?

Example:

Q. What is 35 squared?
A. 3x4=12
.....now stick on the 25
...........the answer is 1225

Do you know why it works?

Times Eleven

The eleven times table has always been very easy to learn up to 9 x 11. Here's a way of multiplying large numbers by 11 too:

Example:

Q. What is 324 x 11 ?
A. Write down the first digit ... 3
.......Add the first and second digits ... 3 + 2 = 5
.......Add the second and third digits .. 2 + 4 = 6
.......Write down the last digit ........... 4

The answer is 3564.

Try it yourself ... multiply 543 by 11

Do you know why it works?

Does it work for all three digit numbers?

Multiplying by Four

Doubling a number is quite easy. We learn doubles as some of our first number facts. So multiplying by four is easily achieved by doubling, then doubling again.

Example:

Q. What is 742 x 4?
A. 742 x 2 = 1484
.....1484 x 2 = 2968

The answer is 2968

Do you know why it works?

Multiplying by Eight

Multiplying by 8 can be achieved by doubling three times:

Example:

Q. What is 742 x 8?
A. 742 x 2 = 1484
.....1484 x 2 = 2968
.....2968 x 2 = 5936

The answer is 5936

Do you know why it works?