A Complete Guide to
Mental Math
Beta Version

×11

Multiply by 11

×9

Multiply by 9

×5

Multiply by 5, 25, 125

×4

Multiply by 4, 8, 16

Same Start, End Sum to 10

_5²

Square Ending in 5

2 & ½

Doubling & Halving

FOIL Method

≈10s

Multiply Near Same Base

a²–b²

Difference of Squares

Compatible Division

½

Divide by 4, 8, 16

L→R

Left-to-Right Addition

≈

Rounding & Compensating

L→R

Left-to-Right Subtraction

≈

Rounding & Compensating

Σ

Sum Between Numbers

%

Percentage Tricks

Quiz

Practice