Use Mental Arithmetic

The following is a sampling of some ideas to make each of the operations easier to do mentally. There are many other ways, and each learner creates their own as they learn to do arithmetic. Keep trying different shortcuts to see what works for you. See also Use Estimating Skills .

Addition

When adding columns such as:

25
34
98

Subtraction

When subtracting numbers, think about what you need to add to the smaller number to arrive at the larger number.

Multiplication

It helps to learn products of multiples of 10.
If you are multiplying

by 10: put 0 on the end
by 100: put 2 zeros on the end
by 1000: put 3 zeroes on the end

For example:

38 X 10 = 380
38 X 100 = 3800
38 X 1000 = 38000

30 X 2 = 60
200 X4 = 800

Division

It helps to know divisibility rules, to know if a number is divisible by another number, to do division.

A number
is divisible by .... if...

2.......... the number ends in 0,2,4,6,8

3.......... the sum of digits is divisible by 3, so is the number

4.......... it is divisible by 2 twice -- if half a number is even

..the last two digits are 0s
..the last two digits can divide by 4 (example 190,732)

5.......... the number ends in 0 or 5

6.......... the number is divisible by 2 and by 3

...it is an even number and
the sum of the digits is a multiple of 3

8.......... the last 3 digits are 0s


..the last three digits can divide by 8 (example 17,720)

9.......... the sum of digits is divisible by 9 (just like 3)

10......... the number ends in 0

11......... all digits are the same and


it has an even number of digits

It also helps to know the products of multiples of 10 when trying to find quotients (in division). For example:

1500 / 20
Divide each number by 10 first (or by 100, etc.)
The problem becomes: 150 / 2
The answer is 75.