**1. Addition of 5**

When adding 5 to a digit greater than 5, it is easier to first subtract 5 and then add 10.

For example,

8 + 5 = 13.

Also 8 - 5 = 3; 3 + 10 = 13.

12 + 5 = 17.

Also 12 - 5 = 7; 7 + 10 = 17.

**2. Subtraction of 5**

When subtracting 5 from a number ending with a a digit smaller than 5, it is easier to first add 5 and then subtract 10.

For example,

26 - 5 = 21.

Also 26 + 5 = 31; 31 - 10 = 18.

23 - 5 = 18.

Also 23 + 5 = 28; 28 - 10 = 18.

23 - 5 = 18.

Also 23 + 5 = 28; 28 - 10 = 18.

**Division by 5**

Similarly, it's often more convenient instead to multiply first by 2 and then divide by 10.

For example,

1375/5 = 2750/10 = 275.

980/5 = 1960/10 = 196.

**Multiplication by 5**

It's often more convenient instead of multiplying by 5 to multiply first by 10 and then divide by 2.

For example,

140×5 = 1400/2 = 700.

135×5 = 1350/2 = 675

120×5 = 1200/2 = 600

it's more quick right ?!

**Division/multiplication by 25**

Use operations with 4 instead.

For example,

37×25 = 3700/4 = 1850/2 = 925.

**Add a sequence from one to a selected**

1-digit number and back

Choose a 1-digit number.

Square it.

Example:

If the 1-digit number selected is 7:

To add 1 + 2 + 3 + 4 + 5 + 6 + 7 + 6 + 5 + 4 + 3 + 2 + 1

Square 7: 49

So the sum of all numbers from 1 through 7 and back is 49.

See the pattern?

If the 1-digit number selected is 9:

To add 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1

Square 9: 81

So the sum of all numbers from 1 through 9 and back is 81.