Mental Math Tricks

Question 0

6x6

Answer 0

36

Question 1

Multiples of 9 (up to 10)

Answer 1

Add up to 9, first digit one less than the multiplier

Question 2

7x6

Answer 2

42

Question 3

6x8

Answer 3

48

Question 4

7x7

Answer 4

49

Question 5

7x8

Answer 5

56

Question 6

8x8

Answer 6

64

Question 7

11x_ _ _

Answer 7

=sum of adjacent digits in between digits of original multiplier (unless 1 carries)

Question 8

Square of 2 digit numbers ending in 5 (35^2)

Answer 8

(first digit) x (n+1), ends in 25 (=1225)

Question 9

Multiplying two digit numbers beginning with the same digit and last digits add to 10 (84x86)

Answer 9

Answers begin with (first digit) x (n+1), and end with the product of the last digits (=7224)

Question 10

Multiply any two numbers between 11-19 (18x14)

Answer 10

Add last digit of the second number to the first number, multiply by 10, then add the product of the last two digits (=18+4,=22x10,=220+32=252)

Question 11

Compliments

Answer 11

First digits (tens, hundreds, thousands, etc.) add to 9, and last (ones place) add to 10 (unless ending in 0)

Question 12

Adding and subtracting up to three digit numbers

Answer 12

Use rounding/compliments then add or subtract value back at end, go from left to right

Question 13

Multiplying 2x1 or 3x1

Answer 13

Multiply 100’s, 10’s, and 1’s then add together (use rounding when possible to make problems easier)

Question 14

Addition Method

Answer 14

Can be used for any multiplication problem but best when one of the numbers ends in a small digit (round down to nearest easy number) 41x17= 17(40+1)= 680 + 17= 697

Question 15

Subtraction Method

Answer 15

Use when one of the numbers ends in a large digit (round up to nearest easy number then subtract difference) 53x89= 53(90-1)= 4770-53= 4717

Question 16

Factoring Method

Answer 16

Factor one of the numbers then multiply by each factor. 73x42= 73x7x6= 511x6= 3066 (good because don’t have to mentally hold on to any numbers)

Question 17

Math of least resistance

Answer 17

Look at problem both ways then take easier path. Try larger factor first but take what is easiest.

Question 18

Temperature conversion

Answer 18

F=1.8C+32 [1.8=(6x3)/10]

Question 19

Squaring #s using close together method

Answer 19

Round to nearest multiple of 10, then -/+ the difference in the opposite direction, finally add the square of the difference. 97^2= 100x94 + 3^2= 9400+9= 9409

Question 20

Multiplying with the close together method

Answer 20

Round one digit to nearest multiple of 10, then -/+ the difference in the opposite direction to the other number, finally add the product of the 2 differences (of original #s to ‘0). *23x26= 20x29 + 3x6= 580+18= 598

• 47x45= 50x42 + (-3)(-5)= 2100+15= 2115
• 43x38= 40x41 + (3)(-2)= 1640-6= 1634

Question 21

Short Division

Answer 21

Done on paper, faster than long division because do more work in your head and write down less.

Question 22

Vedic division

Answer 22

Turns a hard two digit division problem into easy one digit division. Remainder moves up one place and add the (result)(multiplier) to the remainders

Question 23

Major system/ peg system (0-9)

Answer 23

0= s, z/ no peg word
1= t, d/ tea
2= n/ knee
3= m/ moo
4= r/ ear
5= L/ oil
6= ch, sh, j/ shoe
7= k, g/ key
8= f, v/ foe
9=p, b/ pie

Question 24

More peg words (10-13; 40)

Answer 24

10= dice
11= tot
12= tin
13= tomb
40= rose

Question 25

Calendar calculator

Answer 25

Month code + date + year code - 7n

Question 27

Calendar calculator: days

Answer 27

Monday - Saturday = 1- 6

Sunday= 0 or 7

Question 28

Calendar calculator: months

Answer 28

Jan= 6 (5 leap year) 'winter'
Feb= 2 (1 leap year) '2nd month'
March= 2
April= 5
May= 0
June= 3
July= 5
Aug= 1
Sept= 4
Oct= 6
Nov= 2
Dec= 4

Question 29

Calendar calculator: more year codes

Answer 29

1900's= 1
1800's= 3
1700's= 5
1600's= 0
1582= pope Gregory XIII established Gregorian Calendar, universally adopted in 1920's (not always 7 days a week)

Question 30

Calendar calculator: years 2000-2099

Answer 30

=Years + leaps - 7n
(Leap= divisible by 4 or 400, but not 100)
Repeats every 28 years