Secrets ofSecrets ofMental MathMental Math

By: Katie Blumenstock

•Calculations that are either done mentally or with pencil or paper.

•They are tricks to solve what looks to be difficult math equations

•Committed to memory

•Multiplication, Division, or Double facts.

What is Mental Math?What is Mental Math?

What the practice of What the practice of Mental Math can do!Mental Math can do!

Adding NumbersAdding Numbers • From 1st Grade, You were always told to add…

RIGHT TO LEFT! THEY WERE WRONG!!

Adding from Left to Right is the Simplest and Quickest Way!

Adding NumbersAdding Numbers 47

+ 32 (30 + 2)

47 + 32→ 47 + 30 = 77 + 2 = 79

Multiplying by 9 Multiplying by 9

Multiplying By 11 Multiplying By 11

The Quicker Way

3+2 = 5 1. Take the first factor

and add the two numbers together.

2. Split the first factor and place the sum of the factor in the middle

352

Examples 23 x 1145 x 11 27 x 11

Examples Examples 23 x 11 = 25345 x 11 = 495 27 x 11 = 297

There’s an Exception to There’s an Exception to Every RuleEvery Rule

• If the sum of the first factor is greater than 10, you simply carry over…

(6 + 7 = 13)1

+ 637 737

2-by-1 2-by-1 MultiplicationMultiplication

42 x 7

The Quicker Way42x 7

(40 + 2) 1st Break up the first factor

2nd Multiply the small multiplication tasks

40 x 7 = 280 2 x 7 = 14

3rd Add both of the small multiplication tasks together

Examples Examples

87 x 5

38 x 9

Examples 87 x 5

38 x 9

(80 + 7)

80 x 5 = 4007 x 5 = 35400 + 35 = 435

(30 + 8)

30 x 9 = 2708 x 9 = 72 270 + 72 = 342

OverviewOverview• There are many more tricks to Mental Math

– Figuring out the week of any date in the past

• Andriod and IPhone apps

• Book by “Secrets of Mental Math” By Arthur Benjamin and Michael Shermer

References

• Benjamin, Arthur and Shermer, Michael. “Secrets of Mental Math.” Lowell House (2006).