Mental Calculations - Getting the result fast

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,

7 + 5 = 12.

Also 7 - 5 = 2; 2 + 10 = 12.

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,

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.

More examples and explanation

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,

137×5 = 1370/2 = 685.